Carry the preview time to and from .osu files

[MAX-WiRED]

The point a song previews from is lost in both directions when a .osu file is
involved.

Why

PreviewTime is the same value as #SAMPLESTART, counted in milliseconds
rather than seconds. The writer emits a constant:

Write(out, "PreviewTime", "-1");

so a simfile that has a preview loses it on export, and ParseGeneral reads
only AudioFilename and Mode, so a .osu that has one loses it on import.
osu! itself sets the field from its editor, and most ranked maps carry a real
value.

The change

The loader reads it into previewStart, and the writer emits what is stored:

} else if (IsProp(prop, "PreviewTime")) {
    out.previewTime = Str::readInt(PropVal(prop)) * 0.001;
}

Write(out, "PreviewTime",
      sim->previewStart > 0.0 ? ToMilliseconds(sim->previewStart) : -1);

osu! writes -1 for a song with no preview, so that value is left alone on the
way in and still written on the way out when there is nothing to write.

Neither osu! nor a simfile agree on the length: osu! records only where the
preview starts. #SAMPLELENGTH therefore stays at zero on import, which is
what the games fall back to their own default on - the same thing that already
happens today, since nothing was imported at all.

Reproducing

osu file format v14

[General]
AudioFilename: audio.mp3
PreviewTime: 60819
Mode: 3

[Metadata]
Title:Preview
Artist:Test
Version:Normal

[Difficulty]
CircleSize:4

[TimingPoints]
0,500,4,1,0,100,1,0

[HitObjects]
64,192,0,1,0,0:0:0:0:
192,192,500,1,0,0:0:0:0:

Open it and look at File -> Properties: the preview fields are empty. Save it
as .sm and #SAMPLESTART is 0.000000.

Testing

Built on Windows with clang-tidy and clang-format enforced, as the build does.

With the change, that file gives #SAMPLESTART:60.819000;, and saving back to
.osu gives PreviewTime:60819. The same file with PreviewTime: -1 still
gives #SAMPLESTART:0.000000; and is written back as PreviewTime:-1, so a
map with no preview does not gain one at the start of the song.