Migrate to GemsFX Notifications

[calixtus]

Very much WIP

ToDos:

• Fix Show InfoCenter button in MenuBar
• Align at bottom
• Fix background
• Migrate notifications
• Remove additional notification pane over maintable (for external modification notification)
• Investigate if background jobs can be migrated (create custom notification ?)
• Polish

See https://github.com/dlsc-software-consulting-gmbh/GemsFX/blob/master/gemsfx-demo/src/main/java/com/dlsc/gemsfx/demo/InfoCenterApp.java

Steps to test

Run JabRef, do something that produces a notification (e.g. View > Next preview or open a file)

Mandatory checks

• I own the copyright of the code submitted and I license it under the MIT license
• I manually tested my changes in running JabRef (always required)
• [.] I added JUnit tests for changes (if applicable)
• I added screenshots in the PR description (if change is visible to the user)
• [.] I described the change in CHANGELOG.md in a way that is understandable for the average user (if change is visible to the user)
• [.] I checked the user documentation: Is the information available and up to date? If not, I created an issue at https://github.com/JabRef/user-documentation/issues or, even better, I submitted a pull request updating file(s) in https://github.com/JabRef/user-documentation/tree/main/en.

[github-actions]

Your pull request conflicts with the target branch.

Please merge with your code. For a step-by-step guide to resolve merge conflicts, see https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/addressing-merge-conflicts/resolving-a-merge-conflict-using-the-command-line.