Shared preferences editor improvements

[MuntashirAkon]

• Display an option (preferably in a snackbar) to reload the file if it's been modified by another app. Display a warning if there are unsaved changes. The warning may contain the options to merge or discard the changes.
• After the file has been reloaded, highlight the changed item (preferably with the present and the past changes) as well as the newly added items.