Remove m_isSelecting and StartSelecting() from wxGridSelection to avoid
inconsistency of wxGridSelection selecting mode state with wxGrid state,
as wxGrid already has m_isDragging field which tells it if the selection
state is final or not.
Instead, just allow wxGrid to specify the event to send from
ExtendCurrentBlock().
We still need a separate EndSelecting() for sending the final
wxEVT_GRID_RANGE_SELECTED event, but send it only for the last selection
block, and not all the selected blocks, as this makes more sense (there
should be one SELECTED event for each block and it was already sent for
the other blocks before) and is consistent with the events generated
when performing the same actions from keyboard.
0a2c62fc5c