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.bkl

@@ -10,6 +10,7 @@
     <exe id="docview" template="wx_sample" template_append="wx_append">
         <sources>docview.cpp doc.cpp view.cpp</sources>
         <headers>docview.h doc.h view.h</headers>
+        <wx-lib>aui</wx-lib>
         <wx-lib>core</wx-lib>
         <wx-lib>base</wx-lib>
         <win32-res>docview.rc</win32-res>