Fix issue koppor/jabref#521 (Add feature: allow drag&drop from selected bib entries to other library tabs)

[GNAQ]

According to issue#521 from koppor/jabref issue tracker, we added new feature about allowing drag selected entries, and drop them to other opened inactive bib library tabs.

Behavior Description:

Now when dragging selected entries in main table, JabRef allows user to drop it on another opened library tab, and this item will be copied and added to the dropped library accordingly. Drop to current active tab is disallowed. If there're potential duplicated entries, JabRef will ask user to confirm and choose copy behavior (similar to ctrl-c/v copy and paste).

Link issues that are fixed:

Fixed: JabRef#521

Screenshots for UI changes:

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.