RF: Adjust pnm_stage1 web server creation to avoid pickle problems on macOS
- create server objs in server process, so that server objs don't need to be transferred between processes
- Use a mp.Value to send port# back to main process.
- Use a startup event so that main process doesn't attempt to read port# before server has started
Also fix the user specified slice timing file dialog so that it defaults to *.txt
, rather than expecting image file types