[!!!][TASK] Use progress bar instead of mask for loading

The core currently has two places where a loading
indicator is shown:

* Loading iframes, using a mask created by ExtJS
* Loading IRRE elements via AJAX, hand-made for IRRE

This behaviour should be unified with a new and clean API
using the JS library nprogress (MIT license).

Some old CSS code can safely removed, as it is not
used anymore.

Releases: master
Resolves: #64035
Change-Id: Iaf45af96aa022571195d1a27cfaa9dfd0f4ad63a
Reviewed-on: http://review.typo3.org/34242
Reviewed-by: Jigal van Hemert <jigal.van.hemert@typo3.org>
Tested-by: Jigal van Hemert <jigal.van.hemert@typo3.org>
Reviewed-by: Helmut Hummel <helmut.hummel@typo3.org>
Tested-by: Helmut Hummel <helmut.hummel@typo3.org>