Clarify that CPU is between samples and category&stack are at sample time

When CPU numbers are shown, the tooltip looks like this: So it's something like this:

Category: [] Layout... CPU Usage: 97% (...) Stack: parse_into()... ...

I'm concerned that we're mixing two "domains" of information, and it may give the wrong impression, e.g.: "Wow, this parse_into in layout is taking 97% of a CPU!". But in fact, all we can say from this data is: Since the previous sample and now, we have used 97% of a CPU; And at this exact sample time we were calling parse_into in layout.

So I think we should clarify the difference. Suggestion:

CPU Usage since previous sample: 97% (...) ..................................................................................... Sample at time: 0.8465s Category: [] Layout... Stack: parse_into()... ...

This clearly separates the more continuous CPU numbers from the point-like sample data.

As a bonus it shows the precise timestamp. I don't know how valuable that is, but for example it could help find markers at that time in the Marker Table.

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