Use ximerama indices for _NET_WM_FULLSCREEN_MONITORS

Bug: 9351

_NET_WM_FULLSCREEN_MONITORS uses Xinerama indices for multi-monitor
fullscreen, but xfwm4 uses gdk indices which may differ.

As a result, depending on the actual layout and which monitor is marked
as primary, a fullscreen window could be placed on the wrong outputs.

Fetch the Xinerama list and use the monitor size from there instead of
relying on gdk monitor indices for _NET_WM_FULLSCREEN_MONITORS