[TASK] Use FAL function for image rendering

Instead of relying on the backward compatiblity layer of the
FrontendContentAdapterService when rendering images with css_styled_content
the render_textpic method is modified so that it is also be possible
to use FAL functions and properties for image rendering.

The captionsSplit / imageTextSplit constants are removed because every image
has its own properties for that with FAL and they are not needed any more.

The globalCaption rendering was removed because the captions are now
always attached to a single image.

The longdescURL handling was also removed because the files do not have this
property at the moment and the longdescURL field for tt_content records is
also not visible in the Backend.

Resolves: #53764
Releases: 6.2
Change-Id: I1d9c8ad1d7a498816e724960613818a05d587d4f