[BUGFIX] Center legacy extension icons

Add a horizontal margin to typo3-app-icon to ensure legacy extension
icons are centered.

Resolves: #65139
Releases: master
Change-Id: I4669237173194e6c265b2ca40c2736b3c73b12b3
Reviewed-on: http://review.typo3.org/36986


Reviewed-by: Frederic Gaus <gaus@flagbit.de>
Tested-by: Frederic Gaus <gaus@flagbit.de>
Reviewed-by: Jigal van Hemert <jigal.van.hemert@typo3.org>
Tested-by: Jigal van Hemert <jigal.van.hemert@typo3.org>