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/ButtonWidget.cs

@@ -292,13 +292,10 @@ namespace OpenRA.Mods.Common.Widgets
 			if (string.IsNullOrEmpty(baseName))
 				return;
 
-			var variant = highlighted ? "-highlighted" : "";
-			var state = disabled ? "-disabled" :
-						pressed ? "-pressed" :
-						hover ? "-hover" :
-						"";
+			var variantName = highlighted ? baseName + "-highlighted" : baseName;
+			var imageName = WidgetUtils.GetStatefulImageName(variantName, disabled, pressed, hover);
 
-			WidgetUtils.DrawPanel(baseName + variant + state, rect);
+			WidgetUtils.DrawPanel(imageName, rect);
 		}
 	}
 }