Performance improvements in the custom widget

[oldtechaa]

We've been having some trouble with performance in the custom widget, so we need to continue minimizing calls to expose(). For example, when a note is removed, the button handler calls expose(). That needs to be removed by clearing only the affected rectangle and redrawing the grid around it. See Cairo documentation for more details on how to do that.

[oldtechaa]

This issue will no longer be a problem as we switch to Gtk3 #26 with its clip regions by default and we have switched to different methods of drawing on the widget. This can be closed.

[oldtechaa]

Originally, this issue was reopened because we would not switch to Gtk3. We restarted work on Gtk3 and have completed the switch, eliminating most of the need for this issue. However, there are still performance improvements that can be made, such as by switching to a $notes{"$x;$y"}[] HoA with constants for the list indices.

[oldtechaa]

Moved to Milestone 9 because this is not urgently needed and may increase code complexity too much to be worthwhile for the small memory gain.