core: Remove trailing '/' from GrlMedia IDs

grl-launch-0.3 -S browse -k title grlbox://grl-tracker-source/videos/
should have the same output as:
grl-launch-0.3 -S browse -k title grlbox://grl-tracker-source/videos

The first would get its ID parsed to "videos/" whereas the second would
be parsed to "videos". Chomp the trailing '/'.

https://bugzilla.gnome.org/show_bug.cgi?id=758654