Fix use of DELETE and BACKSPACE when starting editing grid with them.

Fix use of DELETE and BACKSPACE when starting editing grid with them.

Using the current insertion position in wxGridCellTextEditor::StartingKey()
didn't make much sense, it was always 0 -- so DELETE worked as expected and
did delete the first character of the cell but BACKSPACE never did anything.

Just always delete the first character when DELETE is used and always delete
the last one when BACKSPACE is.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@65749 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775