Provide auto-layout option for CIM models missing DiagramObjects

As discussed the auto layout itself should be handled by a standalone tool.

Work on this standalone tool has already started: https://git.rwth-aachen.de/acs/core/cim/CIMelk

I imagine a workflow like this:

User opens a CIM file without DiagramObjects
Pintura shows a dialog and informs the user about the missing layout information.
The same dialog offers the user the option to invoke our standalone CIM layouter
Pintura runs the CIMelk layouter in the background
Pintura loads the now layout'ed CIM file
The user can adjust the layout and save the CIM file, now including the new DiagramObjects