* Added undo/redo functionality thanks to Nathan Whitehead.
* Added opacity-set functionality thanks to Lukas Hermann.
* Added a tray icon.
* Added an About and a Help dialog.
* Added CMake build system support thanks to Russel Winder.
* Replaced GTK3 deprecated functions with newer counterparts.
* Added reasonably complete support for slave devices thanks to Monty Montgomery.
* Tweaked build system, packaging, icon thanks to Barak A. Pearlmutter.
* Fixed some crashes.
* Concurrent multi-user operation using GTK3 with Multi-Pointer-X.
* Drawn annotations do not intercept mouse clicks.
* Makes use of the Composite extension where available to allow for
really fast drawing.