Set main stage for file dialogs

[tobiasdiez]

Sets the correct parent for the file dialogs so that these are now modal.

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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• Check documentation status (Issue created for outdated help page at help.jabref.org?)

[Siedlerchr]

I think you have selected the wrong base branch or did you intend to merge it into the telemetry branch?

[stefan-kolb]

Ah this should fix some open issues (or at least one)?!

[tobiasdiez]

@Siedlerchr yeah, my bad. I now changed it to the correct base branch (and now the telemetry commit pops up as a new change...Tobi and git).
@stefan-kolb good observation, I was not aware that this was reported previously. Yes, this PR should fix #3410. I'll add it to the changelog in #3621 as soon as this PR gets merged.