fix: Allow using deprecated color AppSettings

This partiall reverts commit efeb02cd34,
bringing back the `AppSettins::Color*` and making them the backing store
for `App::color`, to help ease the transition from clap2->3.

Once we remove these deprecated settings, we might want to keep this
backing store to save on memory.

This is a part of #2617

src/build/app/mod.rs

@@ -90,7 +90,6 @@ pub struct App<'help> {
     pub(crate) template: Option<&'help str>,
     pub(crate) settings: AppFlags,
     pub(crate) g_settings: AppFlags,
-    pub(crate) color: ColorChoice,
     pub(crate) args: MKeyMap<'help>,
     pub(crate) subcommands: Vec<App<'help>>,
     pub(crate) replacers: HashMap<&'help str, &'help [&'help str]>,
@@ -1021,9 +1020,13 @@ impl<'help> App<'help> {
     /// [`ColorChoice::Auto`]: crate::ColorChoice::Auto
     #[cfg(feature = "color")]
     #[inline]
-    pub fn color(mut self, color: ColorChoice) -> Self {
-        self.color = color;
-        self
+    pub fn color(self, color: ColorChoice) -> Self {
+        #[allow(deprecated)]
+        match color {
+            ColorChoice::Auto => self.setting(AppSettings::ColorAuto),
+            ColorChoice::Always => self.setting(AppSettings::ColorAlways),
+            ColorChoice::Never => self.setting(AppSettings::ColorNever),
+        }
     }
 
     /// Sets the terminal width at which to wrap help messages. Defaults to
@@ -2818,8 +2821,21 @@ impl<'help> App<'help> {
     }
 
     #[inline]
+    // Should we color the output?
     pub(crate) fn get_color(&self) -> ColorChoice {
-        self.color
+        debug!("App::color: Color setting...");
+
+        #[allow(deprecated)]
+        if self.is_set(AppSettings::ColorNever) {
+            debug!("Never");
+            ColorChoice::Never
+        } else if self.is_set(AppSettings::ColorAlways) {
+            debug!("Always");
+            ColorChoice::Always
+        } else {
+            debug!("Auto");
+            ColorChoice::Auto
+        }
     }
 
     #[inline]

src/build/app/settings.rs

@@ -25,6 +25,9 @@ bitflags! {
         const NO_POS_VALUES                  = 1 << 17;
         const NEXT_LINE_HELP                 = 1 << 18;
         const DERIVE_DISP_ORDER              = 1 << 19;
+        const COLOR_ALWAYS                   = 1 << 21;
+        const COLOR_AUTO                     = 1 << 22;
+        const COLOR_NEVER                    = 1 << 23;
         const DONT_DELIM_TRAIL               = 1 << 24;
         const ALLOW_NEG_NUMS                 = 1 << 25;
         const DISABLE_HELP_SC                = 1 << 27;
@@ -56,7 +59,7 @@ pub struct AppFlags(Flags);
 
 impl Default for AppFlags {
     fn default() -> Self {
-        Self::empty()
+        AppFlags(Flags::COLOR_AUTO)
     }
 }
 
@@ -77,6 +80,12 @@ impl_settings! { AppSettings, AppFlags,
         => Flags::ALLOW_NEG_NUMS,
     AllowMissingPositional("allowmissingpositional")
         => Flags::ALLOW_MISSING_POS,
+    ColorAlways("coloralways")
+        => Flags::COLOR_ALWAYS,
+    ColorAuto("colorauto")
+        => Flags::COLOR_AUTO,
+    ColorNever("colornever")
+        => Flags::COLOR_NEVER,
     DontDelimitTrailingValues("dontdelimittrailingvalues")
         => Flags::DONT_DELIM_TRAIL,
     DontCollapseArgsInUsage("dontcollapseargsinusage")
@@ -489,6 +498,18 @@ pub enum AppSettings {
     /// ```
     SubcommandPrecedenceOverArg,
 
+    /// Deprecated, see [`App::color`]
+    #[deprecated(since = "3.0.0", note = "Replaced with `App::color`")]
+    ColorAuto,
+
+    /// Deprecated, see [`App::color`]
+    #[deprecated(since = "3.0.0", note = "Replaced with `App::color`")]
+    ColorAlways,
+
+    /// Deprecated, see [`App::color`]
+    #[deprecated(since = "3.0.0", note = "Replaced with `App::color`")]
+    ColorNever,
+
     /// Disables the automatic collapsing of positional args into `[ARGS]` inside the usage string
     ///
     /// # Examples