Some changes from over Christmas.

Unsuccessful attempt to handle transient/dialog windows better.
Add minimum/maximum size handling for windows.
Add Makefile hook for running karuiwm in Xephyr.

NOTE:
I'm going to rewrite karuiwm from scratch, given the mess I've created
so far, in hope I can correctly handle the transient/dialog window
stuff. Furthermore I'm about to add some form of external communication
capabilities (dbus, unix socket, ...), so some deeper changes will be
necessary.
=> see also changes in dzenstat that are about to happen.