Many deprecation warnings when running Magnus

[kankaristo]

Magnus seems to be using a few deprecated functions. I'm getting the following messages:

/usr/bin/magnus:107: DeprecationWarning: Gdk.Display.get_device_manager is deprecated
  devman = self.w.get_screen().get_display().get_device_manager()
/usr/bin/magnus:108: DeprecationWarning: Gdk.DeviceManager.get_client_pointer is deprecated
  self.pointer = devman.get_client_pointer()
/usr/bin/magnus:294: DeprecationWarning: Gdk.Screen.get_width is deprecated
  sw = float(scr.get_width())
/usr/bin/magnus:295: DeprecationWarning: Gdk.Screen.get_height is deprecated
  sh = float(scr.get_height())
/usr/bin/magnus:233: DeprecationWarning: Gdk.Display.get_pointer is deprecated
  (screen, x, y, modifier) = display.get_pointer()

I'm using the latest version of Magnus (1.0.3) on Ubuntu 20.10 (with Gnome Shell 3.38.1, if that makes a difference).

[stuartlangridge]

These are deprecated, but I don't think I want to rebuild Magnus to use Gdk.Seat and so on quite yet, because then it'll depend on newer Gtk, which might not be ideal. @flexiondotorg can you comment on whether this would be a problem for Ubuntu MATE and snap users? If it isn't a problem then changing things to use Seat is of course doable.