[TASK] Additional SignalSlot after init in EditDocumentController

Adds a new SignalSlot possibility after the init method call.

Resolves: #56381
Releases: 6.2
Change-Id: I2357f81c40b123a7cd2eef57ef142a9e934dbc35
Reviewed-on: https://review.typo3.org/27896
Reviewed-by: Julian Kleinhans
Tested-by: Julian Kleinhans
Reviewed-by: Tomas Norre Mikkelsen
Reviewed-by: Erik Frister
Reviewed-by: Joh. Feustel
Reviewed-by: Stefan Rotsch
Reviewed-by: Anja Leichsenring
Tested-by: Anja Leichsenring