[BUGFIX] Language label and css loading in inline and flex sections

Loading additional stylesheets from elements in inline scope fails if using
'EXT:' syntax, for flex section containers, this was not evaluated at all.
The patch adds code to resolve an EXT: prefix stylesheet file from an element
result array ['stylesheetFiles'] to properly handle loading of this CSS in
JS, and uses it in both inline and flex ajax controller. This issue is easily
reproducible with ext:styleguide 'elements_t3editor', which registers
t3editor.css to load. Without patch, This fail (404) in 'in inline'
't3editor_inline_1' 'Create New'.
Additionally, additional language file handling was not implemented for
flex section container ajax calls, which is also added by the patch,
similar to the solution done in inline ajax.

Change-Id: I617b9f1ed3e0c4718fb0867a42f2c37b604d7afe
Resolves: #80335
Releases: master
Reviewed-on: https://review.typo3.org/52089


Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Frank Nägler <frank.naegler@typo3.org>
Tested-by: Frank Nägler <frank.naegler@typo3.org>
Reviewed-by: Susanne Moog <susanne.moog@typo3.org>
Tested-by: Susanne Moog <susanne.moog@typo3.org>