Allow overriding print preview frame creation in docview.

Add a virtual wxDocManager::CreatePreviewFrame() which can be overridden to
customize the print preview used by docview framework.

Closes #11390.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@63757 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

include/wx/docview.h

@@ -478,6 +478,12 @@ public:
 #endif // WXWIN_COMPATIBILITY_2_6
 
 protected:
+#if wxUSE_PRINTING_ARCHITECTURE
+    virtual wxPreviewFrame* CreatePreviewFrame(wxPrintPreviewBase* preview,
+                                               wxWindow *parent,
+                                               const wxString& title);
+#endif // wxUSE_PRINTING_ARCHITECTURE
+
     // hook the currently active view into event handlers chain here
     virtual bool TryBefore(wxEvent& event);