Various incremental updates to improve PropBASIC coverage.
- Add PropBASIC manual to help menu
- Add support for multiple builders per language definition
- Add *.plib extension for included PropBASIC files
- Report correct extension patterns in open/save dialogs
- Rewrite save dialog to better support multiple filetypes; prevent build
action from disabling when file contents have changed. - Rewrite program logic to clearly delineate on/off states of various file
actions - Clear project view when all files are closed