Create files passed on the command line even when Frescobaldi is already open

[jeanas]

Since #1556, Frescobaldi properly creates a new file if passed a nonexistent file on the command line.

However, I've noticed that this still doesn't work if there is already a Frescobaldi instance when you launch Frescobaldi (which does IPC with the existing instance in that case, to reuse it for opening the files). When doing this, Frescobaldi gives an error about not being able to open the file.