huge change: implement RandR instead of Xinerama

huge change: implement RandR instead of Xinerama

Thanks to Merovius for doing a proof of concept on this one and
being a driving force behind the idea.

Using RandR instead of Xinerama means that we are now able to use
the full potential of the modern way of configuring screens. That
means, i3 now has an idea of the outputs your graphic driver
provides, which allowed us to get rid of the ugly way of detecting
changes in the screen configuration which we used before. Now, your
workspaces should not be confused when changing output modes anymore.

Also, instead of having ugly heuristics to assign your workspaces
to (the screen at position X or the second screen in the list of
screens) you will be able to just specify an output name.

As this change basically touches everything, you should be prepared
for bugs. Please test and report them!