fix: use try_insert instead of insert in bevy_ui to prevent panics when despawning ui nodes (#13000)

# Objective

Sometimes when despawning a ui node in the PostUpdate schedule it
panics. This is because both a despawn command and insert command are
being run on the same entity.

See this example code:

```rs
use bevy::{prelude::*, ui::UiSystem};

#[derive(Resource)]
struct SliceSquare(Handle<Image>);

fn main() {
    App::new()
        .add_plugins(DefaultPlugins)
        .add_systems(Startup, setup)
        .add_systems(Update, create_ui)
        .add_systems(PostUpdate, despawn_nine_slice.after(UiSystem::Layout))
        .run();
}

fn setup(mut commands: Commands, asset_server: Res<AssetServer>) {
    commands.spawn(Camera2dBundle::default());
    commands.insert_resource(SliceSquare(asset_server.load("textures/slice_square.png")));
}

fn create_ui(mut commands: Commands, slice_square: Res<SliceSquare>) {
    commands.spawn((
        NodeBundle {
            style: Style {
                width: Val::Px(200.),
                height: Val::Px(200.),
                ..default()
            },
            background_color: Color::WHITE.into(),
            ..default()
        },
        UiImage::new(slice_square.0.clone()),
        ImageScaleMode::Sliced(TextureSlicer {
            border: BorderRect::square(220.),
            center_scale_mode: SliceScaleMode::Stretch,
            sides_scale_mode: SliceScaleMode::Stretch,
            max_corner_scale: 1.,
        }),
    ));
}

fn despawn_nine_slice(mut commands: Commands, mut slices: Query<Entity, With<ImageScaleMode>>) {
    for entity in slices.iter_mut() {
        commands.entity(entity).despawn_recursive();
    }
}
```

This code spawns a UiNode with a sliced image scale mode, and despawns
it in the same frame. The
bevy_ui::texture_slice::compute_slices_on_image_change system tries to
insert the ComputedTextureSlices component on that node, but that entity
is already despawned causing this error:

```md
error[B0003]: Could not insert a bundle (of type `bevy_ui::texture_slice::ComputedTextureSlices`) for entity Entity { index: 2, generation: 3 } because it doesn't 
exist in this World. See: https://bevyengine.org/learn/errors/#b0003
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
Encountered a panic when applying buffers for system `bevy_ui::texture_slice::compute_slices_on_image_change`!
Encountered a panic in system `bevy_ecs::schedule::executor::apply_deferred`!
Encountered a panic in system `bevy_app::main_schedule::Main::run_main`!
```

Note that you might have to run the code a few times before this error
appears.

## Solution

Use try_insert instead of insert for non critical inserts in the bevy_ui
crate.

## Some notes

In a lot of cases it does not makes much sense to despawn ui nodes after
the layout system has finished. Except maybe if you delete the root ui
node of a tree. I personally encountered this issue in bevy `0.13.2`
with a system that was running before the layout system. And in `0.13.2`
the `compute_slices_on_image_change` system was also running before the
layout system. But now it runs after the layout system. So the only way
that this bug appears now is if you despawn ui nodes after the layout
system. So I am not 100% sure if using try_insert in this system is the
best option. But personally I still think it is better then the program
panicking.

However the `update_children_target_camera` system does still run before
the layout system. So I do think it might still be able to panic when ui
nodes are despawned before the layout system. Though I haven't been able
to verify that.

crates/bevy_ui/src/accessibility.rs

@@ -78,7 +78,7 @@ fn button_changed(
             }
             commands
                 .entity(entity)
-                .insert(AccessibilityNode::from(node));
+                .try_insert(AccessibilityNode::from(node));
         }
     }
 }
@@ -107,7 +107,7 @@ fn image_changed(
             }
             commands
                 .entity(entity)
-                .insert(AccessibilityNode::from(node));
+                .try_insert(AccessibilityNode::from(node));
         }
     }
 }
@@ -137,7 +137,7 @@ fn label_changed(
             }
             commands
                 .entity(entity)
-                .insert(AccessibilityNode::from(node));
+                .try_insert(AccessibilityNode::from(node));
         }
     }
 }

crates/bevy_ui/src/texture_slice.rs

@@ -177,7 +177,7 @@ pub(crate) fn compute_slices_on_asset_event(
             atlas,
             &atlas_layouts,
         ) {
-            commands.entity(entity).insert(slices);
+            commands.entity(entity).try_insert(slices);
         }
     }
 }
@@ -213,7 +213,7 @@ pub(crate) fn compute_slices_on_image_change(
             atlas,
             &atlas_layouts,
         ) {
-            commands.entity(entity).insert(slices);
+            commands.entity(entity).try_insert(slices);
         }
     }
 }

crates/bevy_ui/src/update.rs

@@ -63,7 +63,7 @@ fn update_clipping(
         }
     } else if let Some(inherited_clip) = maybe_inherited_clip {
         // No previous calculated clip, add a new CalculatedClip component with the inherited clipping rect
-        commands.entity(entity).insert(CalculatedClip {
+        commands.entity(entity).try_insert(CalculatedClip {
             clip: inherited_clip,
         });
     }
@@ -163,7 +163,7 @@ fn update_children_target_camera(
 
         match camera_to_set {
             Some(camera) => {
-                commands.entity(child).insert(camera.clone());
+                commands.entity(child).try_insert(camera.clone());
             }
             None => {
                 commands.entity(child).remove::<TargetCamera>();