Simplify API for extending wxListCtrl background display

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.

include/wx/generic/listctrl.h

@@ -189,6 +189,8 @@ public:
     virtual bool SetFont( const wxFont &font ) wxOVERRIDE;
     virtual bool SetCursor( const wxCursor &cursor ) wxOVERRIDE;
 
+    virtual void ExtendRulesAndAlternateColour(bool extend = true) wxOVERRIDE;
+
 #if wxUSE_DRAG_AND_DROP
     virtual void SetDropTarget( wxDropTarget *dropTarget ) wxOVERRIDE;
     virtual wxDropTarget *GetDropTarget() const wxOVERRIDE;
@@ -208,8 +210,6 @@ public:
     wxListHeaderWindow  *m_headerWin;
     wxListMainWindow    *m_mainWin;
 
-    void SetListRulesAlternateColourOnBlank(bool state, const wxColour& colour);
-
 protected:
     // Implement base class pure virtual methods.
     long DoInsertColumn(long col, const wxListItem& info) wxOVERRIDE;