2.91.2:
* Fixed reference counting in completion classes and added a completion test.
  (Krzesimir Nowak)
* Renamed gtksourceview namespace to Gsv.
* Written documetation about migration from 2.0 to 3.0 - it describes most of
  API breaks (like the one mentioned above). (Krzesimir Nowak)
* Wrapped GtkSourceGutterRenderer, GtkSourceGutterRendererPixbuf,
  GtkSourceGutterRendererText and GtkSourceMarkCategory. (Krzesimir Nowak)
* Use the gtk conversions from gtkmm now that they are not in glibmm.
  That bumps mm-common requirement to yet unreleased version. (Murray Cumming)