Added methods to insert MenuItems and MenuItemSeparators into MenuBars.  Also fixed a MenuBar bug where the visible submenu of the previous MenuItem would remain open if the user highlights a MenuItem that does not have a visible submenu.

Patch by: jlabanca
Review by: jgw
Issue: 2301



git-svn-id: https://google-web-toolkit.googlecode.com/svn/trunk@3624 8db76d5a-ed1c-0410-87a9-c151d255dfc7