[MWF] DataGridViewRowCollection Clear() speed up

When issuing a Clear() to remove all rows, a ReIndex() was being called for
each row that was being removed.  This is extremely inefficient, instead
do a single ReIndex() after all rows have been removed.

Before doing a Clear() of 30k rows was taking 5.8 seconds.  With this patch
it now takes 670ms.
Merge pull request #1396 from jwestfall69/dgvrc-clear-speedup

[MWF] DataGridViewRowCollection Clear() speed up