[FEATURE] Add signal slots to SoftReferenceIndex

This patch adds two signal slot dispatch calls to the SoftReferenceIndex
in the getTypoLinkParts() method and the setTypoLinkPartsElement()
methods.

These slots allow third party link handlers to generate their own
soft reference data.

Resolves: #21396
Releases: 6.3, 6.2
Change-Id: If2bbcd1757ed480f31c1bdccd69bba2a77df9a8f
Reviewed-on: https://review.typo3.org/31423
Reviewed-by: Markus Klein
Tested-by: Markus Klein