Replace SetListRulesAlternateColourOnBlank() taking 2 arguments, with
the second of them being used only when the first one is true, with a
simpler but still sufficient ExtendRulesAndAlternateColour(bool).
Make the new method virtual and define it as doing nothing in
wxListCtrlBase class, so that it's still available, even if currently
not implemented, in wxMSW.
Also simplify the implementation, fix style problems and other minor
improvements.
5ae2a8ebb8