Fix window position restore on multi-head setups

Gajim stopped to properly restore roster and dialogs windows positions
after restart with more than one monitor attached.

Gajim saves the absolute window positions, i.e. based on the total screen size
of all monitors attached. Despite Gtk deprecated Gdk.Screen.width() and height(),
we cannot just use width() and height() for one monitor, as recommended.
We still need the total screen size, which was the base for saving the positions.

[1] https://stackoverflow.com/questions/30207586/gdk-screen-vs-monitor-vs-display