Shift+Tab on first field of entry editor should focus last field of previous tab

https://github.com/JabRef/jabref/pull/13938 introduced the functionality to <kbd>Tab</kbd> through all fields of the entry editor.

There should be the inverse functionality: <kbd>Shift</kbd>+<kbd>Tab</kbd> should focus the previous field - and if there is no previous field on the current entry editor tab, focus the previous tab and the last field.

Code can get inspirations from #13938.