Sets the current keyboard focus to be at path, and selects it. This is
usefull when you want to focus the user's attention on a particular item.
If cell is not 0, then focus is given to the cell speicified by it.
Additionally, if start_editing is TRUE, then editing should be started in
the specified cell.
This function is often followed by grab_focus() in order to give keyboard
focus to the widget.