Fix changing colours of wxSpinCtrl in wxMSW

Ensure that the new colours are actually shown after Refresh() is called
by refreshing the text part of the control as well, and not just the
spin button.

Closes https://github.com/wxWidgets/wxWidgets/pull/1468

Closes #12382.

include/wx/msw/spinctrl.h

@@ -65,6 +65,8 @@ public:
     virtual int GetBase() const;
     virtual bool SetBase(int base);
 
+    virtual void Refresh( bool eraseBackground = true,
+                          const wxRect *rect = (const wxRect *) NULL ) wxOVERRIDE;
 
     // implementation only from now on
     // -------------------------------