Don't disable wxStaticBox window label when disabling the box

This behaviour might be not completely intuitive, but it makes it much
simpler to handle the box state using a checkbox as the label control
(which is by far the most common case of using box window labels).

Notice that while we could add a separate EnableWithoutLabel() method to
wxStaticBox to make it possible to set the state of the box directly
relatively easily, it wouldn't help with using wxEVT_UPDATE_UI for
managing the box state indirectly as it relies on calling Enable() only.
And this solution does allow wxEVT_UPDATE_UI handlers for the box itself
to work (provided the handler takes care to check for the event object
being the box itself, as otherwise it would still disable the child
checkbox when its wxEVT_UPDATE_UI bubbles up to the box).

interface/wx/statbox.h

@@ -158,4 +158,30 @@ public:
                 const wxSize& size = wxDefaultSize,
                 long style = 0,
                 const wxString& name = wxStaticBoxNameStr);
+
+    /**
+        Enables or disables the box without affecting its label window, if any.
+
+        wxStaticBox overrides wxWindow::Enable() in order to avoid disabling
+        the control used as a label, if this box is using one. This is done in
+        order to allow using a wxCheckBox, for example, label and enable or
+        disable the box according to the state of the checkbox: if disabling
+        the box also disabled the checkbox in this situation, it would make it
+        impossible for the user to re-enable the box after disabling it, so the
+        checkbox stays enabled even if @c box->Enable(false) is called.
+
+        However with the actual behaviour, implemented in this overridden
+        method, the following code (shown using C++11 only for convenience,
+        this behaviour is not C++11-specific):
+        @code
+            auto check = new wxCheckBox(parent, wxID_ANY, "Use the box");
+            auto box = new wxStaticBox(parent, wxID_ANY, check);
+            check->Bind(wxEVT_CHECKBOX,
+                        [box](wxCommandEvent& event) {
+                            box->Enable(event.IsChecked());
+                        });
+        @endcode
+        does work as expected.
+     */
+    virtual bool Enable(bool enable = true);
 };