Allow tracking arbitrary entities in spatial views

[Wumpf]

In

• https://github.com/rerun-io/rerun/pull/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 #810

[Wumpf]

the PR that fixed this was reverted

[IsseW]

Re-implemented in https://github.com/rerun-io/rerun/pull/11554