Fix for issue koppor/jabref#521 (add new features about drag&drop between bib libraries)

[GNAQ]

According to issue#521 from koppor/jabref issue tracker, we are adding new feature about allowing drag and drop of items between different opened bib libraries.
This PR will be submitted to JabRef when it is ready.

Behavior Description:

Now when dragging an entry of bib library, JabRef allows user to drop it on another opened library tab, and this item will be copied and added to the dropped library accordingly.

Link issues that are fixed:

Fixed: JabRef#521

Screenshots for UI changes:

Waiting for addition.

Checklist / TODOs :

• Code implement of this new feature
• 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 developer's documentation: Is the information available and up to date? If not, I outlined it in this pull request.
• 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.