Simplify LO connection and fix threading errors

[Siedlerchr]

Simplify LO connection

• Test sucessful for mac

@LyzardKing I simplified the LO connection. It's now enough to input the dir of the executable. JabRef now only calls the soffice.exe
The jars are no longer needed.

Edit// Can you test if the connection to a snap LO or so is now possible?

• Change in CHANGELOG.md described (if applicable)
• Tests created for changes (if applicable)
• Manually tested changed features in running JabRef (always required)
• Screenshots added in PR description (for UI changes)
• Checked documentation: Is the information available and up to date? If not created an issue at https://github.com/JabRef/user-documentation/issues or, even better, submitted a pull request to the documentation repository.

[LyzardKing]

I'm testing the flatpak now.. it should work (like the latex editors..)
If I get that working I'll look at the snap, although for that it's a bit more complicated.
In the snap it's not possible to escape the confinement, so we'd need to set it to unconfined (classic)

[LyzardKing]

The flatpak JabRef works with the apt/rpm version of libreoffice.
Is it possible to set the default path to /app/bin/soffice in the flatpak?

EDIT: Is there a file with the configurations that could be edited when building the flatpak?

[Siedlerchr]

@LyzardKing I implemented an approach based on the Env variable you suggested.
to check if FLATPAK_SANDBOX_DIR exists and then return the path /app/bin/soffice

Alternatively, we could add a variable e.g. Linux_Variant to the build.properties with the info if snap or flatpak.
Edit// But that would need to be present in the java compiler step

[LyzardKing]

@Siedlerchr I'll test in the next few days, but this should be enough to have (from the next release) a flatpak that works out of the box with libreoffice/browsers/tex editors

[tobiasdiez]

Can the OO jars now also be removed from the gradle file?

[Siedlerchr]

Can the OO jars now also be removed from the gradle file?

No! The only change is that we no longer need to load the jars from the libre office directory because we already need them for building and running (like other libraries).
That is why I adjusted the LO bootstrap
In addition, I fixed some threading errors for error messages.