Add Monitor_greedyView()

[dbbnrl]

This is a hack to emulate XMonad's greedyView function.

It is similar to Monitor_activateView() but it ensures that there is
only ever one copy of a view (instead of one copy per monitor). When we
switch to view X using Monitor_greedyView(), all windows from view X on
all monitors are moved to view X on the active monitor.

If the view we're switching to is currently visible on another monitor,
they are swapped.

[dbbnrl]

I notice there has been a bit of activity here lately, including some discussion of this PR. I have been using it (old version of bug.n with this PR applied) for several years now. If there is interest in reviving this idea, I'd be happy to spend some time getting this PR back into a usable state with the latest code. If the maintainers would rather not incorporate this feature, I totally understand and I won't bother fixing up the code.

[ghost]

I rather like the idea; it is one of the few, which can be added to bug.n without redesigning the whole view-monitor-model. Therefor I would be glad to incorporate this feature.
Currently I do not have much spare time and therefor will not mess up the code as before ;-) The code base for the pr will be stable for quite a while, giving you enough time, I hope, to adjust your code.

In any case, thank you for your contribution.