The Welcome Tab currently does not include "Import"

Task

• Add "Import into new library..." to Welcome Tab. Same functionality as File > Import > Import into new library
• Rename "Import into new library" to "Import into new library..." in File menu
• Rename "Import into current library" to "Import into current library..." in File menu
• Add three dots to "Export*" in File menu.

Localization hint: https://devdocs.jabref.org/code-howtos/localization.html