Add wxEVT_COLOURPICKER_CURRENT_CHANGED and DIALOG_CANCELLED events

Send events from generic wxColourPickerCtrl when the currently selected
colour in the dialog shown by it changes and when this dialog is
cancelled.

Notice that currently this only works on wxMSW as it relies on
wxEVT_COLOUR_CHANGED support in wxColourDialog which is only available
there.

Based on work of Trylz, see https://github.com/wxWidgets/wxWidgets/pull/1219

include/wx/generic/clrpickerg.h

@@ -15,6 +15,8 @@
 #include "wx/bmpbuttn.h"
 #include "wx/colourdata.h"
 
+class wxColourDialogEvent;
+
 //-----------------------------------------------------------------------------
 // wxGenericColourButton: a button which brings up a wxColourDialog
 //-----------------------------------------------------------------------------
@@ -75,6 +77,8 @@ protected:
     static wxColourData ms_data;
 
 private:
+    void OnColourChanged(wxColourDialogEvent& event);
+
     wxDECLARE_DYNAMIC_CLASS(wxGenericColourButton);
 };