Added a new method SetButtonWidth(UInt_t buttonWidth) for setting
the button width in pixels.

Invoke this method before calling the TControlbar::Show() if you
want to specify the button width for all control bar buttons:
    bar->SetButtonWidth(90);
    bar->Show();


git-svn-id: http://root.cern.ch/svn/root/trunk@19219 27541ba8-7e3a-0410-8455-c3a389f83636