- Main change is the addition of wxGridSelectRowsOrColumns selection mode

- Main change is the addition of wxGridSelectRowsOrColumns selection mode
  (which is still probably buggy, wxGridSelection needs to be reviewed)
- Add XYToCell() overloads returning wxGridCellCoords (instead of modifying the
  argument passed by reference -- where did this come from?) and document them.
- Added GoToCell() which does make the new current cell visible unlike
  SetGridCursor() (which was documented as doing it, but wasn't)
- Changed SetCurrentCell() to only not change the cell if wxEVT_GRID_SELECT_CELL
  it generates is vetoed, not just processed as this seems to make more sense
- Split jumbo (~400 lines) ProcessGridCellMouseEvent() function into chunks
- Add many more comments to make reading this code seem less like puzzle
  solving for the next unfortunate soul to do it

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@55746 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775