Sub-presentation needs to have preview in the main composition view

Qt 3D Studio and QML subpresentations need to have a preview in the editor view so that designers sees the complete picture when editing. Ultimate solution is to have live preview by having a master preview clock that ticks onwards and the main scene and all subpresentation scenes will derive their local clocks from this. This way all components can animate "as they would in runtime scenario". But the minimum requirement is to see at least first frame of content from the sub-presentation.

So the priorities are as follows:

1. Have at least static content visible from sub-presentations in the edit view.
2. Have animations playing back from sub-presentations in the edit view.
3. Optional: Have animations of the sub-presentation sync with the timeline scrub of the main view.