Add UI for precisely picking an exact sequence time

[emilk]

What

• Part of Ability to jump to a specific frame/timestamp in the timeline view #7653

I only implemented it for sequence timelines for now, since I didn't want to parse timestamps in different formats this early in the morning. Hopefully this can still help @teh-cmc.

It also shows how to do it for temporal timelines if someone wants to follow my lead.

Checklist

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[teh-cmc]

With realtime feedback and everything, sweet!

Nit: if you enter <static>, then the field becomes non-editable once again, which is a bit weird, but eh.

[emilk]

if you enter <static>, then the field becomes non-editable once again

So it becomes… static? 😆