Trigger an event on users undelete action success

All other users actions have handy triggers on which you can hook in
other modules. The undelete action should have one too... That's the why
of this commit.
I'm deliberately not sending the BackendUser object along when I trigger
the event because it's a backend specific object and this could
theoretically be handled in frontend modules too. The user's id and
email should be enough in that case.
This fixes some issues with the multiple text box.

=> It now uses .on(), so it will be more compatible with newer versions
of jQuery
=> If multiple entries split by SplitChar are entered into a field,
they get split up and added to the list
PHPDoc fixes
method should not be static
Merge pull request #616 from SpacefulSpecies/master

KernelLoader fixes
Merge pull request #615 from turanct/multiple-textbox-fixes

This fixes some issues with the multiple text box.
Merge pull request #612 from turanct/users-undelete-event

Trigger an event on users undelete action success