Make GNOME Music usable with GTK+ 3.8

**NOTE THAT THIS IS NOT READY FOR MERGE YET**
Might need more changes, details below. Also, for testing purposes :D
Fix for #96.

To @roignac:
* Should I ignore micro versions for GTK+ functions? Setting of custom title bars needs GTK+ 3.9.8. If you agree that I should just ignore them, I'll edit the patches so Gtk.get_minor_version() would be used instead of Gtk.check_version().
* I'm planning to check if this works in GTK+ 3.4 and 3.6 too. (If we could, why not? :D)
* If you want, I'll rename the other ui files too.
* The first commit is actually kind of unrelated. It's actually a fix for #102. Also, it hides the back button when in selection mode to ensure that when we go back to main page of Albums view, the header bar disables selection mode. I included it here because this branch needs it, and it only contains one patch anyway.