Make wxColourData parameter of wxColourDialog ctor const

This parameter is not modified by wxColourDialog (this might have been
the case, or at least the plan, some long time ago) and so has no reason
not to be "const".

Just add the qualifier to ctor and Create() in all ports.

Closes https://github.com/wxWidgets/wxWidgets/pull/1421

Closes #12511.

include/wx/generic/colrdlgg.h

@@ -37,10 +37,10 @@ class WXDLLIMPEXP_CORE wxGenericColourDialog : public wxDialog
 public:
     wxGenericColourDialog();
     wxGenericColourDialog(wxWindow *parent,
-                          wxColourData *data = NULL);
+                          const wxColourData *data = NULL);
     virtual ~wxGenericColourDialog();
 
-    bool Create(wxWindow *parent, wxColourData *data = NULL);
+    bool Create(wxWindow *parent, const wxColourData *data = NULL);
 
     wxColourData &GetColourData() { return m_colourData; }