[FEATURE] Add signal after extension installation

The patch adds a signal after an extension was successfully installed
and all imports and updates are done.

Resolves: #61306
Releases: master
Change-Id: Ifaaf353e44299a830f33d9fe6b818d45e8774fb1
Reviewed-on: http://review.typo3.org/38010


Reviewed-by: Andreas Fernandez <andreas.fernandez@aspedia.de>
Tested-by: Andreas Fernandez <andreas.fernandez@aspedia.de>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>