Feature/#294 untangle open vr

Rundown:

• DisplaySystem is split into a DisplaySystemOpenVR and a DisplaySystemWindow for those two cases. Parent class DisplaySystem.
• The default engine setup creates both
• The Setup class now also sets up default frame graphs that are then passed into the RenderingSystem
• The RenderingSystem still contains some DisplaySystem-specific stuff, which I propose to get rid of in a separate issue as described in the discussion below
• TrackingSystem is renamed to TrackingSystemOpenVR to make the connection clear.

This has not yet been tested on an HMD, since @mbellgardt tested the last version.

@jwendt @ademiralp @mbellgardt