Implement option to use different Flexform Datastructures per type field value (e.g. CType)
Currently it's not possible to make tceforms use different Flexform datastructures for the pi_flexform field based on the Content Type (CType) of the tt_content table because the plugin field ("list_type") is already defined to be the ds_pointerfield in this case.
However, to have Flexform configuration for the "table" and "login" Content Elements as well, it would be necessary to optionally define different DataStructures per CType.
The attached patch therefore introduces the new option "ds_altSrcPointer_perType".
The patch also patches css_styled_content to use that new option instead of overriding the whole "default" DS key.
(issue imported from #M6770)