rgvolume: make tag list writable before modifying it

rgvolume: make tag list writable before modifying it

Making the event itself writable is not enough, it won't make
the actual taglist in the event writable as well. Instead, just
make a copy of the taglist and then create a new tag event from
that if required, replacing the old one. Before we would
inadvertently modify taglists upstream elements might still
be holding on to. Add unit test for this as well.

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