Some more UI updates and fixes.

* Added UICreatureButton class for handling/replacing "battle buttons" (can now be used in other modules).
* Added the ability to use items on creatures via the battle window.
* Some minor cosmetics.

modules/client_skins/skins/default.lua

@@ -12,6 +12,7 @@ local skin = {
 
   styles = {
     'buttons.otui',
+    'creaturebuttons.otui',
     'labels.otui',
     'panels.otui',
     'separators.otui',