Integrate Polarion as "model source"

[MoritzWeber0]

It shall be possible to link a Polarion project as model source to models in the Collaboration Manager.

Some ideas for further projects:

• The integration can be used together with the plugin store to set up a sync from Capella/T4C to Polarion (Capella2Polarion)
• We can list metadata information (number of documents, work items)