Unify widget state image suffixes (disabled, pressed, hover, focus)

- Add a property for arrows image collection (in drop-downs, scrollbars
and production tabs)
- Add a property for separators image collection (in drop-downs)
- Add hover and disable states to the drop-down separator
- Unify button, textfield and checkbox state suffixes

OpenRA.Mods.Common/Widgets/WidgetUtils.cs

@@ -24,6 +24,17 @@ namespace OpenRA.Mods.Common.Widgets
 			return ChromeProvider.GetImage("chrome-" + world.LocalPlayer.Faction.InternalName, name);
 		}
 
+		public static string GetStatefulImageName(string baseName, bool disabled = false, bool pressed = false, bool hover = false, bool focused = false)
+		{
+			var suffix = disabled ? "-disabled" :
+				focused ? "-focused" :
+				pressed ? "-pressed" :
+				hover ? "-hover" :
+				"";
+
+			return baseName + suffix;
+		}
+
 		public static void DrawRGBA(Sprite s, float2 pos)
 		{
 			Game.Renderer.RgbaSpriteRenderer.DrawSprite(s, pos);