Add a new GimpSelectionTool::have_selection() virtual function,
which determines if the image has a selection (default
implementation), or if the tool will create one upon committing
(implemented by subclasses).  Use this function in
gimp_selection_tool_oper_update() to determine the tool function;
in particular, don't use SELECTION_MOVE and SELECTION_MOVE_COPY
when there's no selection.

Override have_selection() in GimpFreeSelectTool, and return TRUE if
we have a polygon with three or more vertices, which will create a
selection upon committing.