PathPattern: rename variable from songpath to track_path
PathPattern: rename variable from songpath to track_path

This class is not coupled to Music contents (as it's indeed going to be
affected soon by changes to fix the path pattern for Video sources wrt
bgo#689123), so the name "songpath" was not a good name.

No change of behaviour in this commit.

stsundermann/banshee,stsundermann/banshee,GNOME/banshee,stsundermann/banshee,GNOME/banshee,Carbenium/banshee,arfbtwn/banshee,stsundermann/banshee,dufoli/banshee,arfbtwn/banshee,babycaseny/banshee,ixfalia/banshee,dufoli/banshee,Carbenium/banshee,ixfalia/banshee,Carbenium/banshee,GNOME/banshee,babycaseny/banshee,arfbtwn/banshee,dufoli/banshee,ixfalia/banshee,dufoli/banshee,arfbtwn/banshee,arfbtwn/banshee,babycaseny/banshee,Carbenium/banshee,ixfalia/banshee,arfbtwn/banshee,GNOME/banshee,stsundermann/banshee,GNOME/banshee,babycaseny/banshee,stsundermann/banshee,GNOME/banshee,ixfalia/banshee,babycaseny/banshee,stsundermann/banshee,stsundermann/banshee,dufoli/banshee,GNOME/banshee,GNOME/banshee,arfbtwn/banshee,babycaseny/banshee,dufoli/banshee,ixfalia/banshee,dufoli/banshee,ixfalia/banshee,Carbenium/banshee,Carbenium/banshee,ixfalia/banshee,babycaseny/banshee,arfbtwn/banshee,dufoli/banshee,babycaseny/banshee