2.91.1:
* Lots API/ABI breaks - a document about migrating from gtksourceviewmm-2.0
  to gtksourceviewmm-3.0 is to be written, because some of ABI breaks are nasty.
  (Krzesimir Nowak)
* SourceBuffer: Wrapped "bracket-matched" signal.
  (Krzesimir Nowak)
* SourceIter: Gone.
  (Krzesimir Nowak)