* New function: overlay text duration in seconds.
`Overlay_DisplayText` took a float as an argument, with no clear indication of what that float represented. That float also evaluates to different durations given different frame rates. However, since we know what the frame rate should be at any given time (since it's stored in a CVar) we can calculate what this duration should be based on the number of seconds we want the overlay to be displayed. That's what `Overlay_DisplayText_Seconds` does.
* Adds Slider for Overlay Sequence Name Durations
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