Fixes a bug in MenuBar where the selectedItem was not set to null when an item is removed or clearItems is called.  This could result in a JS null pointer exception if garbage collection runs because we try to access an element that no longer has any references to it.

Issue: 2178
Patch by: jlabanca
Review by: bruce



git-svn-id: https://google-web-toolkit.googlecode.com/svn/trunk@2115 8db76d5a-ed1c-0410-87a9-c151d255dfc7