[TASK] Resolve cross dep between EXT:backend and EXT:feedit

This patch moves frontend-editing intialisation from the
EditDocumentController (EXT:backend) to a signal listener
in EXT:feedit. Step towards extracting EXT:feedit to TER.

Change-Id: I09a7e5a2205e7020e7145023d7d63c61d9be508c
Resolves: #80244
Releases: master
Reviewed-on: https://review.typo3.org/52011


Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Tymoteusz Motylewski <t.motylewski@gmail.com>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>
Reviewed-by: Georg Ringer <georg.ringer@gmail.com>
Tested-by: Georg Ringer <georg.ringer@gmail.com>