- TGPictureButton - automatic generation of picture correspondent
   to disabled  state added.

- TRootBrowser
    o correct browser history when object which was browsed was deleted.
    o  Alt-RighArrow,  Alt-LeftArrow handling added,  which correspond to
        "Forward", "Back"  browsing history actions.

- gallery.root - renewed with new contrasted thumbnails


git-svn-id: http://root.cern.ch/svn/root/trunk@11904 27541ba8-7e3a-0410-8455-c3a389f83636