Baloo file process seems to be left over if you kill Xournal++

[WillNilges]

Might be a fluke, but from memory of last time I worked on this, a process called baloo_file will linger and continue to hog CPU on your computer. It just happened again. I guess this is my reminder to make sure that this app gets killed/crashes cleanly.

To Repro:

1. Start a transcription
2. Close xournal++ while the plugin is transferring data over to it
3. Open a task manager and observe high CPU usage.