Make sure all users of GtkCMCList do not blindly assume that a button-press-event...

Make sure all users of GtkCMCList do not blindly assume that a button-press-event happens inside of the tree view.

If clicking on the header row, the event's x,y coordinates
were pointing to a different spot than what the event handler
code expected, since those coordinates are relative to the
relevant GDK window, and header row has its own. Calls to
gtk_cmclist_get_selection_info() or gtk_sctree_is_hot_spot()
in such cases therefore returned bogus info.

This fixes things like folderview expanding/collapsing
selected folder when double-clicking its header column, or
selecting first row when single-clicking it.