[BUGFIX] Improve domObjectId splitting for inline in flexform

We use a regex to retrieve relevant informations for the handling
of flexform datastructures and inline records. It failed for some
use cases like deeply nested structures.
A rework of the regex and some additional unit tests stabilize
this fragile area.

Resolves: #76268
Relates: #73004
Releases: master, 7.6
Change-Id: I4f2afed7f07b3f4f0346bf7a63541e8313f7a992
Reviewed-on: https://review.typo3.org/48502


Reviewed-by: Wouter Wolters <typo3@wouterwolters.nl>
Reviewed-by: Jan Rowold <j.rowold@neusta.de>
Tested-by: Jan Rowold <j.rowold@neusta.de>
Reviewed-by: Susanne Moog <typo3@susannemoog.de>
Tested-by: Susanne Moog <typo3@susannemoog.de>