Add a “document will delete” callback function

Frameworks/document/src/document.cc

@@ -601,6 +601,7 @@ namespace document
 	document_t::~document_t ()
 	{
 		D(DBF_Document, bug("%s\n", display_name().c_str()););
+		_callbacks(&callback_t::document_will_delete, this);
 		documents.remove(_identifier);
 	}
 

Frameworks/document/src/document.h

@@ -136,6 +136,7 @@ namespace document
 
 			virtual ~callback_t () { }
 			virtual void handle_document_event (document_ptr document, event_t event) = 0;
+			virtual void document_will_delete (document_t* document) { }
 		};
 
 		void add_callback (callback_t* callback)         { _callbacks.add(callback); }