ui: gtk: Fix some deprecation warnings

All device manager APIs are deprecated now. Much of our usage is
just to get the current pointer, so centralize that logic and use
the new seat APIs

Signed-off-by: Cole Robinson <crobinso@redhat.com>
Message-id: d6dec24220a4e1449a0172119c10c48e145c0f6f.1462557436.git.crobinso@redhat.com
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>