Make searching and substring filtering two separate operations

This might already be covered by other issues. I'm filing it again just to be sure.

I think typing in the search textbox should not affect which samples are being shown in the call tree. We should only highlight matches in the call tree. As a bonus, we could also show back/forward buttons to jump between different matches.

And there should be a separate button to turn the search string into a search filter. Clicking this button would add a filter to the filter stack.

The main advantage of this is that it would preserve ordering between search filters and transforms. The other advantage is that it would make the interaction I'm proposing in #849 less awkward, because you would not need to clear the search field after adding the collapse transform.

Having a fixed ordering between call tree transforms and search filters avoids the following problem, which Jeff recently encountered on The Joy of Profiling:

1. Search for something.
1. Identify a node that is matched by the search filter, go to a descendant of that node, and click the "focus on subtree" button on it.
1. You now have an empty call tree, because none of the descendants of the focused node match the search filter.

┆Issue is synchronized with this [Jira Task](https://mozilla-hub.atlassian.net/browse/FP-714)