Add wxGrid::FreezeTo() method which allows to freeze the given number of
columns and/or rows at the beginning of the grid, i.e. keep them pinned
in place while the rest of the grid is scrolled.
The main wxGridWindow (m_gridWin) now corresponds to the non-frozen part
of the grid, with up to 3 new similar windows for the frozen
rows/columns and the frozen corner cells (which only exist if both rows
and columns are frozen) being additionally used.
Doing this involved adding "wxGridWindow*" parameter to many functions
that previously only worked with m_gridWin itself and addressing
additional complications, such as mouse events that can now cross
different windows.
See https://github.com/wxWidgets/wxWidgets/pull/952 for the original
version of the changes.
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