The situation where a 'Session_policy' is constructed for a label with
no matching policy is in almost all cases a configuration problem.
A diagnostic message eases pin-pointing such mistaks. By adding the
message to the 'Session_policy', servers don't need to manually handle
the exception to provide diagnostic information. This simplifies the
server code in many components.
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
