[FEATURE] Add SignalSlot for ImportService

Resolves: #61780
Change-Id: Ib9e9b55b7ee892929b9bff1fd79bcb49f3a1c784
Reviewed-on: http://review.typo3.org/32926
Reviewed-by: Georg Ringer <georg.ringer@gmail.com>
Tested-by: Georg Ringer <georg.ringer@gmail.com>