<fTrem> sample

[th-we]

We don't have a sample with an <fTrem> element yet. I'm not entirely clear about how to use them.

Per the specs they may be put inside <beam> elements, but they may also occur directly inside <layer>. In my understanding, those kind of tremolos must always be beamed when their total duration is an eighth or shorter, and never be beamed when they are longer (not counting the slashes as beams). Therefore, wrapping them into a <beam> element seems redundant, but might be needed for recording info like @slope.

What way of encoding should we promote by putting it into the sample set?

_{(Maybe we should introduce @slope on <fTrem> as it's also a sensible attribute when there's no beam, just slashes.)}