Experiment with showing the call node's source code line timings

At the moment, when you open the source view for a node in the call tree, the source view displays "global" timings next to each line of source code. This includes _all_ the time spent in the clicked function, not just the time for the clicked call node.

As an alternative, we could display the line timings just for the selected call node. So, as you change the selection in the call tree, the displayed timings in the source view would change.

It's hard for me to predict which mode would be more useful, and in what situations I would use which mode.
To experiment, we could expose both modes in the UI: `Timings: [x] Global [ ] Selected call node`

┆Issue is synchronized with this [Jira Task](https://mozilla-hub.atlassian.net/browse/FP-506)