core: Only allow Pause() when state is PLAYING
core: Only allow Pause() when state is PLAYING

cf. AVTransport:2 documentation, section 2.5.1, Figure 1 and section
2.4.11.

jku/rygel,GNOME/rygel,GNOME/rygel,Pelagicore/Media-Manager-Rygel-patched,Tarnyko/rygel,jku/rygel,Tarnyko/rygel,GNOME/rygel,Pelagicore/Media-Manager-Rygel-patched,Pelagicore/Media-Manager-Rygel-patched