Sub second marker timestamp precision

[WithoutPants]

Resolves #5382

Allows specifying marker times by frame within seconds:

Frames are specified with a trailing .ffff which is the 0-based frame number.

Times with a fractional component are formatted as [hh:]mm:ss.ffff - frame number 12 at time 3:45 is displayed as 3:45:0012. The time field accepts the same value without the leading zeroes as well, ie 3:45.12, but will be re-formatted to the other format.

Times at frame 0 are accepted and displayed without the .ffff suffix.

Times from the current frame of the video will include sub-second precision.

The length of the ffff field was an arbitrary choice and I'm happy to hear alternatives.

This PR will be rebased once #5311 is merged.

[feederbox826]

I personally don't prefer frames in SMPTE, I much prefer HH:MM:SS.ms since you

1. avoid frame -> ms conversion, also dealing with DF/NDF (and video.currentTime is .ms)
2. are able to get sub-frame precision easily
3. .ms remains consistent across framerates and conversions
4. makes semantic sense if finely adjusting it, since DF wraparound would be at 29.97 and jump to 00

If you do want to stick to SMPTE, then I suggest sticking with HH:MM:SS:FFFF to not confuse it with .ms

[damontecres]

I also think that more users would be familiar with millisecond times over a frame number.

And not exactly relevant, but just as another data point for reference: Android media3's ExoPlayer uses a millisecond precision 64 bit integer for timestamps when playing video files.

[WithoutPants]

Thanks for the feedback. I've updated to use hh:mm:ss.ms instead. It simplified the code doing it that way anyway.