Add context menus providing more navigation options

[dominikh]

Left-clicking on a goroutine ID currently opens a goroutine window. We should also provide a context menu that allows the user to scroll to or zoom to the goroutine instead. For timestamps, the user could choose between setting the start or the end of the timeline to the timestamp.

A goroutine span's context menu could let the user jump to the matching span in the processor view, for example.