Step to next visible data

[kpreid]

The arrow buttons / keys are currently described as:

• Step back to previous time with any new data (left arrow)
• Step forwards to next time with any new data (right arrow)

I think it would be more useful if they stepped to the next/previous visible change in the data — that is, they should take into account only data which is displayed by the current set of visible space views.

---

I took a look at implementing this myself, but couldn't figure out what pieces to put together to achieve it. re_viewer::App::run_time_control_command() would need to use the blueprint to make a suitable query to the data store to pass enough information to the TimeControl.