[TASK] Refactor backend session watcher

Refactor the ExtJS based session watcher that displays dialogs if
the backend session of a user is about to expire or had expired.
The rewritten module uses jQuery and TWBS modals. If the backend
is currently not active (user is in another tab / window), a Web
Notification is displayed, if the used browser supports it.

Also, some dead code in AjaxLoginHandler.php gets removed.

Releases: master
Resolves: #62722
Change-Id: Ic4aae1d775d0dc0e7390f7ba08ca2280334856f0
Reviewed-on: http://review.typo3.org/33917
Tested-by: Markus Klein <klein.t3@reelworx.at>
Reviewed-by: Markus Klein <klein.t3@reelworx.at>
Reviewed-by: Wouter Wolters <typo3@wouterwolters.nl>
Tested-by: Wouter Wolters <typo3@wouterwolters.nl>