Add multi monitor support for the GTK UI

Currently only --x-output works, as the offset part of the --geometry
parameter is ignored for the moment.
Use relative to the window input coordinates in GTK UI

The old way was using relative to the root window coordinates which
resulted in incorrect coordinates when dealing with virtual desktops,
which in turn resulted in incorrect calibration values.

The rational behind directly using the relative to the window
coordinates is that the GTK window is always kept in fullscreen on the
screen of the touch device.
Remove spaces