Fixes Issue #1030. Converts Tree's internal images to use an ImageBundle and deprecates setImageBase and getImageBase (though they still function properly for backwards compatibility). It also adds an example to the doc to illustrate how to override Tree's default images. Review by: bruce git-svn-id: https://google-web-toolkit.googlecode.com/svn/trunk@1053 8db76d5a-ed1c-0410-87a9-c151d255dfc7