[FEATURE] Prepare generic persistence for overlay handling by signals

To get a better grip on languages and workspaces, we decided
to separate their handling from the storageBackend.

This is the first step, introducing the signals needed to
invoke external action.

In some cleanup it also straightens up the names of the
member variables.

Resolves: #51106
Releases: 6.2
Change-Id: Ie4a79270c5fe5f559f0950ea157c137c5c9cccd1
Reviewed-on: https://review.typo3.org/23128
Reviewed-by: Stefan Neufeind
Tested-by: Stefan Neufeind