xconfig: Change the titlebar if using Qt3

Qt4 is now used by default and will get more testing. In case someone
still uses Qt3 and reports a bug, make it easy to recognize that this is
Qt3.

Acked-by: Alexander Stein <alexander.stein@informatik.tu-chemnitz.de>
Signed-off-by: Michal Marek <mmarek@suse.cz>