[FEATURE] Confirm modal on closing an editform with unsaved changes

A new functionality is introduced that asks the editor to confirm
closing an editform with unsaved changes.

Resolves: #65996
Releases: master
Change-Id: I8b606ec0635690ab2b05624412887b6d2fa7c9c5
Reviewed-on: http://review.typo3.org/38134


Reviewed-by: Mathias Schreiber <mathias.schreiber@wmdb.de>
Tested-by: Mathias Schreiber <mathias.schreiber@wmdb.de>
Reviewed-by: Daniel Sattler <sattler@b13.de>
Tested-by: Daniel Sattler <sattler@b13.de>
Reviewed-by: Anja Leichsenring <aleichsenring@ab-softlab.de>
Tested-by: Anja Leichsenring <aleichsenring@ab-softlab.de>