Since __*ArrayList*__ isn't safe like __*Vector*__, I'm also adding `synchronized ()` at places where either of its structure's **size()** changes.
Those 6 methods are: **add()**, **addDrawable()**, **remove()**, **removeDrawable()**, **clear()** and **clearDrawable()**.
Of course, all loops traversing this class still needs to be synchronized externally as well; either over **get()** or **getDrawables()**.
Last word: This patch is just a standalone performant refactoring. It's not obligatory for __*ControllerGroup*__'s previous patch.