Children are disposed by Component.

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

Review URL: https://codereview.appspot.com/7229047

git-svn-id: http://blockly.googlecode.com/svn/trunk@606 c400ca83-b69d-9dd7-9705-49c6b8615e23
blockly.css should only be loaded once.  Provide a means for others to wait for its load.

While looking at componentization of Blockly.Toolbar, I realized that it also needed to listen for css loading.
This CL should make it easier for anyone to listen to the singleton loading of css.

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

Review URL: https://codereview.appspot.com/7225052

git-svn-id: http://blockly.googlecode.com/svn/trunk@607 c400ca83-b69d-9dd7-9705-49c6b8615e23