Instead of handling WM_DISPLAYCHANGE on every GdkWindow, only handle
it on an ad-hoc hidden window we create when opening the display.
This has two reasons:
1) we want emit the display::size-changed signal even if there are no
gtk windows currently open
2) we want to emit the signal just once and not once for every window
https://bugzilla.gnome.org/show_bug.cgi?id=757324