XMP

[koppor]

Improves the handling of XMP data. This enables clean PDFs.
A first, very minimal, step to fix https://github.com/koppor/jabref/issues/337.

It's about reading/writing these things here:

• 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 bigger UI changes)
• Checked documentation: Is the information available and up to date? If not: Issue created at https://github.com/JabRef/user-documentation/issues.