Implement enabled flag for fps overlay (#15246)

# Objective

Fixes #15223 

## Solution

Adds an `enabled` flag to the `FpsOverlayConfig` resource with a system
that detects it's change, and adjusts the visibility of the overlay text
entity.

## Testing

I extended the `fps_overlay` example with the option to toggle the
overlay. Run with:
```
cargo run --features="bevy_dev_tools" --example fps_overlay
```

crates/bevy_dev_tools/src/fps_overlay.rs

@@ -5,12 +5,14 @@ use bevy_asset::Handle;
 use bevy_color::Color;
 use bevy_diagnostic::{DiagnosticsStore, FrameTimeDiagnosticsPlugin};
 use bevy_ecs::{
+    change_detection::DetectChangesMut,
     component::Component,
     query::With,
     schedule::{common_conditions::resource_changed, IntoSystemConfigs},
     system::{Commands, Query, Res, Resource},
 };
 use bevy_hierarchy::{BuildChildren, ChildBuild};
+use bevy_render::view::Visibility;
 use bevy_text::{Font, Text, TextSection, TextStyle};
 use bevy_ui::{
     node_bundles::{NodeBundle, TextBundle},
@@ -47,7 +49,7 @@ impl Plugin for FpsOverlayPlugin {
             .add_systems(
                 Update,
                 (
-                    customize_text.run_if(resource_changed::<FpsOverlayConfig>),
+                    (customize_text, toggle_display).run_if(resource_changed::<FpsOverlayConfig>),
                     update_text,
                 ),
             );
@@ -59,6 +61,8 @@ impl Plugin for FpsOverlayPlugin {
 pub struct FpsOverlayConfig {
     /// Configuration of text in the overlay.
     pub text_config: TextStyle,
+    /// Displays the FPS overlay if true.
+    pub enabled: bool,
 }
 
 impl Default for FpsOverlayConfig {
@@ -69,6 +73,7 @@ impl Default for FpsOverlayConfig {
                 font_size: 32.0,
                 color: Color::WHITE,
             },
+            enabled: true,
         }
     }
 }
@@ -119,3 +124,15 @@ fn customize_text(
         }
     }
 }
+
+fn toggle_display(
+    overlay_config: Res<FpsOverlayConfig>,
+    mut query: Query<&mut Visibility, With<FpsText>>,
+) {
+    for mut visibility in &mut query {
+        visibility.set_if_neq(match overlay_config.enabled {
+            true => Visibility::Visible,
+            false => Visibility::Hidden,
+        });
+    }
+}

examples/dev_tools/fps_overlay.rs

@@ -19,6 +19,7 @@ fn main() {
                         // If we want, we can use a custom font
                         font: default(),
                     },
+                    enabled: true,
                 },
             },
         ))
@@ -47,7 +48,8 @@ fn setup(mut commands: Commands) {
             c.spawn(TextBundle::from_section(
                 concat!(
                     "Press 1 to change color of the overlay.\n",
-                    "Press 2 to change size of the overlay."
+                    "Press 2 to change size of the overlay.\n",
+                    "Press 3 to toggle the overlay."
                 ),
                 TextStyle::default(),
             ));
@@ -62,4 +64,7 @@ fn customize_config(input: Res<ButtonInput<KeyCode>>, mut overlay: ResMut<FpsOve
     if input.just_pressed(KeyCode::Digit2) {
         overlay.text_config.font_size -= 2.0;
     }
+    if input.just_pressed(KeyCode::Digit3) {
+        overlay.enabled = !overlay.enabled;
+    }
 }