Introduce a few pixels of error margin in sessionrc, otherwise the
tests are too fragile, widgets have a tendency to change size a bit
and window managers have a tendency to move windows a bit.
Still needs more work, should probably do the same on book positions
too...
In GIMP 2.6 dockrc did not contain the factory entries for the session
infos, so set that up manually if needed. Also take the opportunity to
add a copyright notice to dialogs.c since tha file is almost rewritten
by now...