taglist: add guard to check writability when removing tags from a taglist
taglist: add guard to check writability when removing tags from a taglist

https://bugzilla.gnome.org/show_bug.cgi?id=762793

StreamUtils/gstreamer,centricular/gstreamer,centricular/gstreamer,centricular/gstreamer,centricular/gstreamer,StreamUtils/gstreamer,StreamUtils/gstreamer,centricular/gstreamer,StreamUtils/gstreamer,StreamUtils/gstreamer