[FEATURE] Root line consideration when building FCE lists

This commit solves a very important lacking feature: building of available FCEs based on multiple root lines. The change adds the following support:

* All TS templates, not just root templates, are now taken into consideration when building the list of available Fluid content elements. Which means you can include sets of templates absolutely anywhere the page tree.
* Conditions are used when configuring the list of available elements, displaying only the appropriate set or sets as specified by the root line.
* Multiple inheritance. If two or more TS templates exist in the root line, content element collections and -configurations are merged with inheritance setup.
* The change is reflected in the new content wizard as well as the "Fluid content type" selector box.