Implement wxDocument::Revert() and show its use in the sample.

wxDocument::Revert() method existed but didn't do anything, provide a
default implementation for it. Also document it and show it in the sample.

Closes #11849.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@63748 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

interface/wx/docview.h

@@ -1332,6 +1332,14 @@ public:
     */
     virtual bool SaveAs();
 
+    /**
+        Discard changes and load last saved version.
+
+        Prompts the user first, and then calls DoOpenDocument() to reload the
+        current file.
+    */
+    virtual bool Revert();
+
     //@{
     /**
         Override this function and call it from your own SaveObject() before