[libbanshee] Only pass playbin's state-changed messages to managed code
[libbanshee] Only pass playbin's state-changed messages to managed code

Previously, all state-changed messages on the bus would be passed to the managed state_changed_cb.  The managed code only cares about the state of the playbin; it doesn't need to know when each individual element in the pipeline changes state.
This change is correct for trunk, but is more important for the gapless branch, where the extra state-changed messages confuse things.

directhex/banshee-hacks,Dynalon/banshee-osx,ixfalia/banshee,babycaseny/banshee,ixfalia/banshee,arfbtwn/banshee,Dynalon/banshee-osx,babycaseny/banshee,arfbtwn/banshee,GNOME/banshee,stsundermann/banshee,directhex/banshee-hacks,ixfalia/banshee,arfbtwn/banshee,babycaseny/banshee,mono-soc-2011/banshee,mono-soc-2011/banshee,Carbenium/banshee,Carbenium/banshee,petejohanson/banshee,GNOME/banshee,petejohanson/banshee,GNOME/banshee,arfbtwn/banshee,dufoli/banshee,allquixotic/banshee-gst-sharp-work,stsundermann/banshee,arfbtwn/banshee,dufoli/banshee,stsundermann/banshee,babycaseny/banshee,allquixotic/banshee-gst-sharp-work,dufoli/banshee,dufoli/banshee,mono-soc-2011/banshee,babycaseny/banshee,mono-soc-2011/banshee,ixfalia/banshee,petejohanson/banshee,lamalex/Banshee,stsundermann/banshee,GNOME/banshee,petejohanson/banshee,Dynalon/banshee-osx,stsundermann/banshee,mono-soc-2011/banshee,dufoli/banshee,ixfalia/banshee,allquixotic/banshee-gst-sharp-work,allquixotic/banshee-gst-sharp-work,Dynalon/banshee-osx,arfbtwn/banshee,dufoli/banshee,Carbenium/banshee,lamalex/Banshee,babycaseny/banshee,mono-soc-2011/banshee,lamalex/Banshee,Dynalon/banshee-osx,lamalex/Banshee,stsundermann/banshee,petejohanson/banshee,lamalex/Banshee,Carbenium/banshee,mono-soc-2011/banshee,dufoli/banshee,directhex/banshee-hacks,GNOME/banshee,GNOME/banshee,GNOME/banshee,arfbtwn/banshee,petejohanson/banshee,directhex/banshee-hacks,directhex/banshee-hacks,arfbtwn/banshee,ixfalia/banshee,Carbenium/banshee,ixfalia/banshee,allquixotic/banshee-gst-sharp-work,Dynalon/banshee-osx,dufoli/banshee,babycaseny/banshee,lamalex/Banshee,Dynalon/banshee-osx,GNOME/banshee,Carbenium/banshee,babycaseny/banshee,Dynalon/banshee-osx,allquixotic/banshee-gst-sharp-work,stsundermann/banshee,ixfalia/banshee,stsundermann/banshee,directhex/banshee-hacks