Allow tracking arbitrary entities in spatial views

[Wumpf]

In

• Double-clicking an entity in the blueprint & time panels focuses the 3D camera on it #4799

all the necessary code for tracking (i.e. continuously refocusing on) entities was added, but we ended up not enabling it on double click:
The issue is that it's very intransparent when tracking is happening and there's nothing indicating the state:

• Make tracked entity an actual part of the camera that can be set from blueprint components
• Indicate ad-hoc tracking somehow?
• add a button upon selecting an entity to make it tracking (and revisit the decision to only foucs on double click)
  • related to Selection of object in 3d scene should have a button to focus on object #810

[Wumpf]

the PR that fixed this was reverted

[IsseW]

Re-implemented in #11554