Include custom device symbols in project backups

PTW allows one to create custom device symbols using Symgen.exe, supplied with PTW. However, these custom symbols are not included in project backups. This creates problems and difficulties when the project is opened up on a different PTW installation than where the custom element was created and used. Backups are not true backups. Those viewing the project on other installations of PTW will not see the same one line as seen by the project creator.