Fixes some bugs in CellBrowser.  CellBrowser#setChildState() was exited early too aggressively, which could put CellBrowser in an invalid state if you mix leaf and non-leaf nodes. CellBrowser was overriding onFocus() to open children, but it isn't focusable if keyboard selection is disabled.  We now override a package protected method that is triggered on focus or mousedown. Also, we would apply the keyboard selected style to the 0th row if keyboard selection is disabled.  This bug has been fixed.  I wrote tests for the mixed leaf/non leaf node cases, and I manually verified that the CellBrowser behaves correctly whether or not keyboard selection is enabled.

Review at http://gwt-code-reviews.appspot.com/942801

Review by: rice@google.com

git-svn-id: https://google-web-toolkit.googlecode.com/svn/trunk@8917 8db76d5a-ed1c-0410-87a9-c151d255dfc7