Added ForceRefresh() to request immediate repainting of a grid.

Calling Refresh() doesn't do this because of our minimal OnPaint
handler (to reduce screen flicker).

Setting of position coords for wxGridEvents now takes account of
row and col label sizes.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@7798 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775