Allow using AUI-based MDI classes in the docview sample

For testing, allow using wxAuiMDI{Parent,Child}Frame in the sample.

This change also shows that the AUI classes are now sufficiently
compatible with the standard ones for switching to them to be almost
trivial.

samples/docview/docview.h

@@ -27,6 +27,9 @@ public:
 #if wxUSE_MDI_ARCHITECTURE
         Mode_MDI,   // MDI mode: multiple documents, single top level window
 #endif // wxUSE_MDI_ARCHITECTURE
+#if wxUSE_AUI
+        Mode_AUI,   // MDI AUI mode
+#endif // wxUSE_AUI
         Mode_SDI,   // SDI mode: multiple documents, multiple top level windows
         Mode_Single // single document mode (and hence single top level window)
     };