Improve "scroll to goroutine" and processor

[dominikh]

When selecting "scroll to goroutine" from a span's context menu, the goroutine should be scrolled to the Y position of the activity we interacted with, not the top of the screen. That way, the user can immediately interact with it, without having to move the mouse.

Need to consider how this should work with the first couple activities in the timeline; we'd need a gap at the top to move them further down.