Refined the fullscreen focus constraints.

Refined the fullscreen focus constraints.

Basically, a focus change can't escape a fullscreen container. The
only exception is per-output fullscreen containers, as you should
be able to focus a container in a different workspace in this case.

This is an improvement on 4eab046e, now considering the difference
between global and per-output fullscreen and taking the tree
structure into account to determine what escaping the fullscreen
container means. It only affects targeted focus commands in the
form "for_window [...] focus", but it lays the foundation for
forthcoming fixes to all other focus commands.