[!!!][TASK] EXT:form - outsource labels and legends to own partials

Move labels and legends to their own partials. This step is slightly
(but thankfully) breaking. The patch allows to avoid duplicate/
redundant code and helps the integrator to override the labels/ legends
with just one small override.

Resolves: #72310
Releases: master
Change-Id: I21781728ac8f35075d05c3cd9df23465d2347abf
Reviewed-on: https://review.typo3.org/45350


Reviewed-by: Bjoern Jacob <bjoern.jacob@tritum.de>
Tested-by: Bjoern Jacob <bjoern.jacob@tritum.de>
Reviewed-by: Wouter Wolters <typo3@wouterwolters.nl>
Reviewed-by: Benni Mack <benni@typo3.org>
Tested-by: Benni Mack <benni@typo3.org>