feat(styles): Allow native style usage to be overridden per control

Add a configuration hook to allow individual control types to opt in/out of using native styling.

This configuration hook is intended to more easily support the most common usage pattern amongst published Uno apps: the UWP styling is used by default for most controls, but the native style is used for a few controls which are optimized for the native style (eg Frame, CommandBar).

src/Uno.UI/FeatureConfiguration.cs

@@ -230,6 +230,16 @@ public static class Style
 			/// a native counterpart. (e.g. Button, Slider, ComboBox, ...)
 			/// </summary>
 			public static bool UseUWPDefaultStyles { get; set; } = true;
+
+			/// <summary>
+			/// Override the native styles usage per control type.
+			/// </summary>
+			/// <remarks>
+			/// Usage: 'UseUWPDefaultStylesOverride[typeof(Frame)] = false;' will result in the native style always being the default for Frame, irrespective
+			/// of the value of <see cref="UseUWPDefaultStyles"/>. This is useful when an app uses the UWP default look for most controls but the native
+			/// appearance/comportment for a few particular controls, or vice versa.
+			/// </remarks>
+			public static IDictionary<Type, bool> UseUWPDefaultStylesOverride { get; } = new Dictionary<Type, bool>();
 		}
 
 		public static class TextBlock

src/Uno.UI/UI/Xaml/Style/Style.cs

@@ -157,7 +157,7 @@ public static void RegisterDefaultStyleForType(Type type, StyleProviderHandler s
 		/// <summary>
 		/// Returns the default Style for given type. 
 		/// </summary>
-		internal static Style GetDefaultStyleForType(Type type) => GetDefaultStyleForType(type, Uno.UI.FeatureConfiguration.Style.UseUWPDefaultStyles);
+		internal static Style GetDefaultStyleForType(Type type) => GetDefaultStyleForType(type, ShouldUseUWPDefaultStyle(type));
 
 		private static Style GetDefaultStyleForType(Type type, bool useUWPDefaultStyles)
 		{
@@ -189,5 +189,15 @@ private static Style GetDefaultStyleForType(Type type, bool useUWPDefaultStyles)
 
 			return style;
 		}
+
+		private static bool ShouldUseUWPDefaultStyle(Type type)
+		{
+			if (type != null && FeatureConfiguration.Style.UseUWPDefaultStylesOverride.TryGetValue(type, out var value))
+			{
+				return value;
+			}
+
+			return FeatureConfiguration.Style.UseUWPDefaultStyles;
+		}
 	}
 }