display: Add vfunc for get_monitor_at_window

display: Add vfunc for get_monitor_at_window

Some backends (namely Wayland) do not support global coordinates so
using the window position to determine the monitor will always fail on
such backends.

In such cases, the backend itself might be better suited to identify
the monitor a given window resides on.

Add a vfunc get_monitor_at_window() to the display class so that we can
use the backend to retrieve the monitor, if the backend implements it.

https://bugzilla.gnome.org/show_bug.cgi?id=766566