Made Workspace a goog.ui.Component

Also cleaned up some dispose methods converting them to disposeInternal and subclassing from goog.Disposable if needed.  Not a complete overhaul, but only the classes that needed to adjust their dispose methods (Flyout, Mutator & Trashcan e.g.)

TBR=neil.fraser@gmail.com
BUG=104

Review URL: https://codereview.appspot.com/7220050

git-svn-id: http://blockly.googlecode.com/svn/trunk@602 c400ca83-b69d-9dd7-9705-49c6b8615e23