[BUGFIX] Restore support for Update script

The new EM is missing support for the class.ext_update.php script.
This patch restores it, giving it its own icon in the extension
list view.

Change-Id: Id5ea243bb689882959aa85cdbe700e784d83b2c0
Resolves: #43244
Releases: 6.1, 6.0
Reviewed-on: https://review.typo3.org/16780
Reviewed-by: Philipp Gampe
Tested-by: Philipp Gampe
Reviewed-by: Anja Leichsenring
Tested-by: Anja Leichsenring
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn