Port techtalk-pse to use GTK3

Should be functionally the same, with two caveats

It is not possible to use the screenshot fallback anymore since
Wayland doesn't permit taking screenshots via GDK APIs. You need
to call the compositor's DBus API. This is what 'gnome-screenshot'
already does, so just always rely on that existing.

For unknown reasons, the 'move' request is never honoured on the
top level window when in non-fullscreen mode.

Signed-off-by: Daniel P. Berrange <berrange@redhat.com>