X11 improvements, fixes, drop SDL

This is maintained drop SDL branch + lots of other improvements and fixes.
I have not submitted this earlier because the code needs some cleanup and I was busy with audio. In order to avoid further confusion I would like to communicate that there was ongoing work on X11 behind the scenes. Thinking of it I have to say that it would be kind of silly not considering this work for Gotham. It is much better than what we have in current master.

- supports multiple monitors on a single X screen
- supports monitors on multiple X screens
- xrandr screen rotations (initial trigger for this endeavor)
- fallback to other monitor if e.g. projector or AVR disconnects
- auto switch to desired monitor when AVR get connected
- fix VideoReferenceClock for multiple monitors
- fix VideoReferenceClock for systems having no nvidia-settings (OpenElec)
- add some missing keys like XF86XK_Sleep

Note that this code has been used by OpenElec for more than a year. Also most AMD users on Linux have tested this over months.

@smspillaz would be great if you could comment this version.