Override DoEnable() instead of Enable() in wxGTK controls

This is slightly simpler, as it doesn't require checking whether the
control state really changes or not (it always does if DoEnable() is
called) and allows disabling the controls before creating them, e.g.
code like

    wxButton* const b = new wxButton();
    b->Disable();
    b->Create(this, wxID_OK);

works as expected now instead of spewing GTK+ errors.

include/wx/gtk/spinbutt.h

@@ -44,8 +44,6 @@ public:
     static wxVisualAttributes
     GetClassDefaultAttributes(wxWindowVariant variant = wxWINDOW_VARIANT_NORMAL);
 
-    virtual bool Enable( bool enable = true ) wxOVERRIDE;
-
     // implementation
     int m_pos;
 
@@ -56,6 +54,8 @@ protected:
     virtual wxSize DoGetBestSize() const wxOVERRIDE;
     virtual GdkWindow *GTKGetWindow(wxArrayGdkWindows& windows) const wxOVERRIDE;
 
+    virtual void DoEnable(bool enable) wxOVERRIDE;
+
 private:
     typedef wxSpinButtonBase base_type;