Use colors to convey the meaning of values in the calltree

In the [hotspot tool](https://github.com/KDAB/hotspot], the values are also displayed using a colored background. The colored background itself had a width that depends on the value. This is nice because this stays accessible: the absolute value are still displayed, and the color isn't the only thing that conveys the meaning, as the width changes too.


┆Issue is synchronized with this [Jira Task](https://mozilla-hub.atlassian.net/browse/FP-736)

Start with the profiler's call tree view and compare its current value presentation with the linked Hotspot example. Done means the absolute values remain visible while color and background width also convey their relative meaning in an accessible way.

Written by the indexing model from the issue text.