With this patch, the wm accounts RAM and caps consumed on behalf of its
clients to the respective client's session quota instead of paying out
of its own pocket. This should make the wm resilient against resource
exhaustion and lowers the quota requirements.
Issue #5340
This commit moves the window layouter and window decorator into
dedicated packages that can now be combined with the "wm" server at
runtime and restarted/reconfigured/swapped-out independently.
To use the window manager, one must start the 'wm', 'window_layouter',
and one of the 'motif_decorator' or 'themed_decorator' subsystems.
Fixes#3024
