Make the selection duration directly editable as a text

I often compare two profiles, opened in adjacent two tabs.
The current workflow is the following:
1. Load the profiles in those two tabs
2. Click "FirstContentfulPaint" in the Marker Chart, so that the selection starts at "Navigation::Start"
4. Move the end of the selection by dragging the marker, so that:
* The duration matches between those two tabs, as much as possible
* The part that I want to compare is within the range
4. Click the magnify icon
5. Maybe do this again if the duration couldn't set to the same value. this can happen if:
* The range that I want is very narrow compared to the full range, and moving the marker by 1px jumps too much and there's no point where it matches between those two tabs
* Simply because moving the pointer by 1px is difficult

It would be nice if the workflow can be improved, for example to the following, by making the duration directly editable as a text:
1. Load the profiles in those two tabs
2. Click "FirstContentfulPaint" in the Marker Chart
3. Directly modify the duration shown at the top of the page (the "3.6s" part of the screenshot. make it an input field)
4. Hit Enter key

┆Issue is synchronized with this [Jira Task](https://mozilla-hub.atlassian.net/browse/FP-955)