Integrated CustomizeExternalFileTypesDialog into Preferences

[calixtus]

Instead of opening the dialog with a button inside the External programs tab in the preferences nested inside the Push to applications preferences, this dialog has now it's own tab in the preferences.

• Change in CHANGELOG.md described in a way that is understandable for the average user (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, I created an issue at https://github.com/JabRef/user-documentation/issues or, even better, I submitted a pull request to the documentation repository.

[Siedlerchr]

Might check if this als fixes #5902

[calixtus]

Probably not, as i only integrated the dialog in the preferences dialog, but i did not change anything on the table.

[calixtus]

But this rather seems to be a javafx bug.

[Siedlerchr]

Merge after 5.4 release due to l10n change