Add "copy to" context menu option for entries in maintable

[HoussemNasri]

Why?

To implement #6404, we need to track the source and destination libraries at the code level when copying entries.

Currently, tracking the state of a copy-paste operation across libraries is not straightforward. When a user copies an entry (ctrl+c) and pastes it (ctrl+v) into another library, JabRef copies the BibTeX data. This process loses any reference to the original library, and there is no way to track where the entry came from or establish cross-referencing between entries.

While workarounds do exist, some of which are mentioned in #6404 (comment), all of them introduce new problems; therefore, we need an additional copy-pasting mechanism/option.

How?

Add a context menu option called copy to to the RightClickMenu.

This option will open a submenu listing all currently open libraries in JabRef, excluding the active one. Selecting a library from the submenu will copy the selected entry to that library.

Copying entries is not always straightforward. For example, when a potential duplicate in the destination library the duplicate resolver dialog will be triggered. Luckily, that code is implemented and it is a matter of figuring out where and how to call it.

Note: We do not need to handle cross-references yet.

Definition of Done

+ Unit tests to ensure the correct libraries are displayed in the submenu.

[yashpandey06]

@HoussemNasri can you help assign this issue ...as per our discussion this is the first task of the series .