Quality names for video streams

The code looks OK but the design is questionable – it takes up a lot of space and doesn't feel very intuitive to use.
Perhaps we can change the "quality label" to something like "Label for this video's quality level" and remove the description as it's very redundant now.

I also suggest that we add a semantics option to specify which fields actually need this label since IV is the only content type which uses them. It feels so wrong that this shows up for H5P.Audio.

Not sure if there's an easy way to fix it, but these fields are so confusing when using a YouTube URL. Is there any way we could hide them?

Ok, so what needs doing:

1. Label to "Label for this video's quality level"
2. Change the description to:
"These labels will be shown in the video quality selector. Example labels may be "1080p" and "720p" or "HD" and "Mobile""
3. Add a semantics option to specify which fields actually need this label since IV is the only content type which uses them.

The YouTube concern has been raised as a separate issue ()

1. Label changed

2. Description changed

3. Added new semantics field "enableCustomQualityLabel" that needs to be true to show quality input fields. Otherwise show old solution (but with larger thumbnails).

NB: Should update documentation on h5p.org with info about the new "enableCustomQualityLabel" semantics field.

Looks and works well. Merge into master. Good job!