[BUGFIX] selicon_field does not create thumbnails

A select field with renderType selectSingle can point to a
foreign_table. If on the foreign_table TCA the ctrl keys
selicon_field and selicon_field_path are set, a single foreign row
can have an "icon" field that is shown to represent this row.

The patch fixes a bug where selicon_field of the own table
instead of the foreign table was used.
Additionally, in master the icon path resolving changed
with #74157 leading to broken icon path in this case. This is
fixed along with a similar change in backend layout code.

Resolves: #75577
Related: #74157
Releases: master, 7.6
Change-Id: I50bac28018b17a61a334aac7d241bcdd96663656
Reviewed-on: https://review.typo3.org/47648


Reviewed-by: Wouter Wolters <typo3@wouterwolters.nl>
Tested-by: Wouter Wolters <typo3@wouterwolters.nl>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>