Add AppThemeColor

[PureWeen]

Description of Change

This adds an AppThemeColor that can be used to drastically simplify creating a color palette and reduce the verbosity needed when creating an AppThemeBinding. NOTE This does make a minor deviation from the original proposal as XamlC was thinking the expected type was AppThemeColorExtension instead of AppThemeColor when declaring the AppThemeColor.

<!-- APP THEME COLORS -->
<AppThemeColor Light="#B0150303" Dark="#FCFCFC" x:Key="TextPrimaryColor" />

<Style x:Key="Headline" TargetType="Label">
    <Setter Property="FontFamily" Value="Segoe UI" />
    <Setter Property="FontSize" Value="10" />
    <!--<Setter Property="TextColor" Value="{AppThemeBinding Light={StaticResource LightTextPrimaryColor}, Dark={StaticResource DarkTextPrimaryColor}}" />-->
    <Setter Property="TextColor" Value="{AppTheme TextPrimaryColor}" />
</Style>

Also added in this PR is AppThemeResource which is meant to be a generic version of the AppThemeColor that could be use more broadly for things like an Image Source which may need to change based on Light/Dark mode:

<ResourceDictionary>
  <AppThemeResource Light="MyLightLogo.png" Dark="MyDarkLogo.png" x:Key="MyLogo" />
</ResourceDictionary>

<Image Source="{AppTheme MyLogo}" />

Issues Fixed

Fixes dotnet/maui#2597

[VladislavAntonyuk]

I like it, but before moving forward, In order to avoid reoccurring issues, what was the reason to remove it from Xamarin.Forms?

[PureWeen]

@jfversluis I'm having a little trouble following the journey of AppThemeColor in Xamarin.Forms. Did we used to have this in XF and then we removed it? It seems like it was around in 4.6 and then removed in 4.7?

[jfversluis]

My initial implementation had it. There was the AppThemeColor and OnAppTheme<T>. Later this was renamed to AppThemeBinding and these helper objects were removed.

Here is AppThemeColor and here is OnAppTheme from the history. These are not available in the current version of Xamarin.Forms anymore.

[jfversluis]

Created a draft PR with a PoC based on Dan's work here: #1264

Seems like there is nothing blocking us from adding this to the Toolkit in its current form. Just need to see if everyone is on board, add some samples and put it in the right place.

[TheCodeTraveler]

Completed #1264