Let dock windows have proper GimpDialogFactory entries. This allows us
to get rid of a lot of ugly mostly duplicated code. This also makes us
ready the merge the dock window and toolbox factories which will soon
be done. A few things should be noted:

 * We adjust the wrap box aspect ratio in the toolbox to avoid having
   the toolbox dock window explode

 * We make sure that we still can handle sessionrc files from GIMP 2.6
   and older