deal with external editor which forks

[mytbk]

For editors such as gvim, the process first forks and exits, so the QProcess state will be:
NotRunning -- (run gvim) --> Starting --> Running --> Not running (clear the tmp file, and the new process edits this file) -- (run second time) --> Starting --> Running --> Not running (and send the file edited last time).

[mytbk]

For gvim, we need to use gvim --nofork

And we shouldn't write the BOM to the tmp file.