[RFC] Bringing back dropsdl branch

@ping elupus
@ping memphiz
@ping montellese

This brings back elupus' shelved "dropsdl" branch and rebases it on master. The commits need some squashing and there are some stray commits, hence the reason why this is an RFC, but I wanted to see how complete this was and where it needed to go from here.

I've fixed some fullscreening and resolution change issues on gnome-shell (turns out that gnome-shell ignores what monitor your window is on when full-screening, you need to set _NET_WM_FULLSCREEN_MONITORS in order to get the behavior you want).

Other than that, see the original description - this branch rids us of SDL for windowing and event handling on X11.