X11: Fix fullscreen toggle

X11: Fix fullscreen toggle

1. Fix fullscreen toggle for window managers that do not have multimonitor fullscreen extension support
2. Fix current monitor detection
3. Fix calculation of vscreen boundaries when single monitor is being used
4. Fix start up position of window when starting (used to always go to the top left corener, now centered)

Still a problem:

1. Window decorations do not show when going windowed
2. Smart resizing makes i3 really sad :(
3. Moving window across monitors and going fullscreen always maximizes on startup screen (when not using /multimon)