add WM_WINDOW_ROLE to selection and check windows

 When there are two top-level windows with the same WM_NAME, WM_CLASS
 and WM_COMMAND values, there needs to be a differing WM_WINDOW_ROLE
 property to distinguish the windows.  We give the WM_S%d selection
 owner window the role "selection" and the _NET_SUPPORTING_WM_CHECK
 window the role "check".