Migrate screen size calculations

[otsaloma]

nfoview/window.py:274: DeprecationWarning: Gdk.Screen.width is deprecated
  size[0] = min(size[0], int(0.8 * Gdk.Screen.width()))
nfoview/window.py:275: DeprecationWarning: Gdk.Screen.height is deprecated
  size[1] = min(size[1], int(0.8 * Gdk.Screen.height()))

[otsaloma]

Seems to require a dependency bump to Gtk/Gdk 3.22. Closing the issue, this is likely best left to be done as part of an eventual GTK+4 migration along whatever other stuff needed.

display = Gdk.Display.get_default()
monitor = display.get_primary_monitor()
geometry = monitor.get_geometry()
scale_factor = monitor.get_scale_factor()
width = scale_factor * geometry.width
height = scale_factor * geometry.height