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/Settings.cs

@@ -176,6 +176,8 @@ namespace OpenRA
 		public Hotkey ObserverWorldView = new Hotkey(Keycode.EQUALS, Modifiers.None);
 
 		public Hotkey TogglePixelDoubleKey = new Hotkey(Keycode.PERIOD, Modifiers.None);
+
+		public Hotkey DevReloadChromeKey = new Hotkey(Keycode.C, Modifiers.Ctrl | Modifiers.Shift);
 	}
 
 	public class IrcSettings