PlayQueue: fix another GtkCRITICAL at shutdown (bgo#710266)

This is the third warning/GtkCritical of the similar kind as the
ones raised in bgo#710266, which was also caused when closing
Banshee after pausing a video. This time the fix is similar to
the previous one[1]: unsubscribing from an event at Dispose time,
which was causing this particular stacktrace: [2].

[1] https://git.gnome.org/browse/banshee/commit/?id=dae6be2d04662b4f9c17261213c394aa635580bf
[2] https://bugzilla.gnome.org/show_bug.cgi?id=710266#c2