[FEATURE] EXT: extensionmanager PNG icon

Use transparent PNG version of backend module icon for
EXT: extensionmanager in module menu instead of GIF.

Change-Id: I424a082c6a294be5f49d4c2a9dd976cb014d5096
Resolves: #51014
Releases: 6.2
Reviewed-on: https://review.typo3.org/23046
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn
Reviewed-by: Wouter Wolters
Tested-by: Wouter Wolters