Add gtk3 support

[LyzardKing]

This is a WIP to solve #5298

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• Change in CHANGELOG.md described
• Tests created for changes
• Manually tested changed features in running JabRef
• Screenshots added in PR description (for bigger UI changes)
• Ensured that the git commit message is a good one
• Check documentation status (Issue created for outdated help page at help.jabref.org?)

[LyzardKing]

@Siedlerchr Do I have to change something in the build system?

[Siedlerchr]

Well, gradle is still used for compiling and building. I am unfortunately not sure how and if this jlink stuff is needed for running

[LyzardKing]

Could we loop @tobiasdiez in this work?
Is there any difference in the build/run of the linux builds?

[Siedlerchr]

Failed to generate snap metadata: The specified command 'desktop-launch' defined in the app 'jabref' does not exist or is not executable.
Ensure that 'desktop-launch' is installed with the correct path.

https://build.snapcraft.io/user/JabRef/jabref/670638

[LyzardKing]

I'll try and run the exact build (in lxd) like the server did.

[tobiasdiez]

I have no experience, with snapcraft so I cannot say much about that. For the build, it should look as follows:

• Run gradle jlink. This gives you a build/image folder containing a working runtime image (JabRef + all dependencies including the necessary java libraries).
• In image/bin, you find some (platform-dependent) luncher scripts (on Windows: JabRef.bat).

So the snapcraft build should bundle the image folder together (or directly use the zip created by gradle jlinkzip) and invoke the correct lunch script (or replicate what they contain).

[LyzardKing]

Thanks! I'll try a few changes now...