[Extensions] move Abstract Content classes to ContentModule

[craigh]

\Zikula\ExtensionsModule\ModuleInterface\Content\AbstractContentType
\Zikula\ExtensionsModule\ModuleInterface\Content\Form\Type\AbstractContentFormType

belong in the ContentModule

Interfaces are welcome but any implementation should be in the proper module.

[craigh]

@Guite I wonder if this may be a reason to file Core RC2 and move them now.

[Guite]

This does not work. See #4354 for how it works.

[craigh]

summary of @Guite's concerns:

• The Interface must be present in the core or extensions containing concrete implementations are broken unless the ContentModule is present/installed. (This is true for other core contracts like BlockInterface)
• The Abstract must be present because it is a lot of code which would be redundant in multiple implementations throughout multiple Content-providing modules.
• Therefore ContentModule becomes a non-required dependency for modules like News.

I believe we need a better solution. I'm not sure that a Contracts package (refs #4354) is an appropriate solution as Contracts are Interfaces only.

[Guite]

Closing as unneeded as content stuff is gone.