Grid accessibility: incorrect focus can be read

Problem: this affects NVDA and Narrator, but not Jaws. If a user moves to the Delete button and presses it to delete a label, a label is incorrectly read as the new focus - the focus remains on the Delete button. GridAx::SetCurrentCell() can be called when the Grid is not the focus, and send a focus event. Fix: In GridAx::SetCurrentCell() only send an focus event if the Grid is the focus.