Start Xinerama implementation (intermediate commit).

Add Monitor structures and put selmon-> everywhere (it's just
intermediate, to make it compile, I'm sure there are bugs all over the
place).

Start with screen detection in Xinerama case.

This thing does not work at all (I guess, I haven't tested yet).
Make it almost work without Xinerama...

Yes, this sounds backwards, given that it worked *without* Xinerama and
the goal is to make it work *with* Xinerama, too.

But given the recent transition to the Monitor structure, there were
lots of things that stopped working, especially with the static monitor
etc. So the first step was to make at least the one-monitor-no-Xinerama
case work, and apart from some warning messages it seems to works quite
well.

The diff is one big chaos, and I have no clue what's been changed.
Fix cleanup(), update README.

It already contains the description for Xinerama, but Xinerama actually
hasn't been tested yet ^^.
Intermediate commit (it works, bitches!).

... actually I don't know - I just managed to handle two displays
separately correctly about two minutes ago, but it crashed after I moved
to the same workspace with both monitors under some circumstances that I
am yet unable to grasp.

But I still thought this is worth a commit (the code is so chaotic the
diffs don't tell anything useful anyway).

Anyway, stay tuned for a more definite commit tomorrow, I'm going to
sleep.
Fix the crash reason (I hope).

Upon calling move(), the focus wasn't correctly updated on the
destination screen, which resulted in a workspace without any focused
window. Calling move() again from that very workspace then resulted in
a segfault or BadWindow at some place.

Update README.
Handle collision prevention.

... and it hasn't crashed yet (will test tomorrow at the Geekerie).

Fix README, add key for changing focus from one monitor to the other.

Only problem is that an empty monitor will make the focus jump to
another monitor's window, which is a bit irritating (maybe I can set the
focus to the root window if there is no window).
Handle detached monitors.

Windows that were displayed on the detached window where "lost", in the
sense of: stwm had no control over them, yet they were still around and
functional.
Merge branch 'xinerama'.

Adds multi-monitor support.
Update README.