Add developer shortcut to reload chrome files

Hitting Ctrl+Shift+C will re-initialize the chrome provider,
reloading all chrome files instantly. Useful when changing the
UI.

Note: A new "RootWidget" was created to trap top-level (global)
shortcuts instead of putting everything in Widget.

OpenRA.Game/Graphics/ChromeProvider.cs

@@ -27,6 +27,8 @@ namespace OpenRA.Graphics
 		static Dictionary<string, Sheet> cachedSheets;
 		static Dictionary<string, Dictionary<string, Sprite>> cachedSprites;
 
+		static string[] storedChromeFiles;
+
 		public static void Initialize(params string[] chromeFiles)
 		{
 			collections = new Dictionary<string, Collection>();
@@ -34,7 +36,13 @@ namespace OpenRA.Graphics
 			cachedSprites = new Dictionary<string, Dictionary<string, Sprite>>();
 
 			if (chromeFiles.Length == 0)
-				return;
+			{
+				chromeFiles = storedChromeFiles;
+				if (chromeFiles == null || chromeFiles.Length == 0)
+					return;
+			}
+			else
+				storedChromeFiles = chromeFiles;
 
 			var chrome = chromeFiles.Select(s => MiniYaml.FromFile(s)).Aggregate(MiniYaml.MergeLiberal);