Introduce document_t::detach_backup

This will reset the document’s backup path but not delete the backup on disk (which can be found via the document’s identifier).

This is useful if we wish to dispose the document but reconstruct it later (e.g. during application termination).

Frameworks/document/src/document.h

@@ -212,6 +212,7 @@ namespace document
 		void try_save (document::save_callback_ptr callback);
 		bool save ();
 		bool backup ();
+		void detach_backup () { _backup_path = NULL_STR; }
 
 		void set_path (std::string const& newPath);
 		void set_virtual_path (std::string const& virtualPath)    { _virtual_path = virtualPath; }