[TASK] favicon for backend window

Adds favicon to the backend view.
Also brings few CGL improvements.

Fixes: #37868
Releases: 6.0
Change-Id: I7b0e71e6b15f8105973ca738a29d95407827b895
Reviewed-on: http://review.typo3.org/11896
Reviewed-by: Wouter Wolters
Tested-by: Wouter Wolters
Reviewed-by: Georg Ringer
Tested-by: Georg Ringer
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn