implement scope rulers and group tracks

[Ichthyostega]

As Lumiera user,
I want the ability to arrange material in a group of tracks,
so I can define some properties of placement, and enable and route output from this group

GUI Implementation task

We can assume that content is already attached into sub-tracks in the core model; how this happens and how content is even attached to some scope in the fork is another topic; more specifically, the UI representation has to follow and reflect the core model completely as it is: If the core attaches some content directly to a track, it must be visible in the UI, even if at the same time there are nested sub tracks — typically there would be either one or the other, but the UI layout must be able to handle both.

Note: as of 1/2023, the details of expanding / collapsing tracks still need some clarification, see #1265

Acceptance Criteria

Given

a Lumiera GUI connected to a session

When

the session contains a track with content placed to nested sub tracks

Then

• either the presentation state of this track is marked as collapsed
  
  • ⟹ in this case only a scope ruler shall be presented
• or the track is marked as expanded
  
  • ⟹ then the track and all sub-tracks shall be presented
    • all rulers of the track other than the scope ruler
    • direct contents of the track if present, otherwise this area shall be reduced
    • an inset or similar stylistic element to mark the scope holding the sub-tracks
    • recursive display of each sub-track
• all Track-Head representation in the »patchbay« shall always follow suite