Completely overhauled selection handling in wxGrid.
Make various ways of extending selection (using Shift-arrow keys,
Ctrl-Shift-arrows, Shift-click etc) work as expected from the user point
of view instead of producing various bizarre results. Also improve
row/column header click selection as well as Ctrl/Shift-Space handling.
Internally, store selection as just a vector of blocks, independently of
the selection mode, and provide a simple API for iterating over it which
remains usable even with selections containing millions of cells (as
long as they're still composed of only a few blocks, which is the case
in practice).
Add more tests and add display of the current selection to the sample.
See https://github.com/wxWidgets/wxWidgets/pull/1772
a5a7641616