Setting playback speed should save META_SPEED

The following discussion from !2092 (merged) should be addressed:

• @Skantes started a discussion:

  META_SPEED is only saved once saveMediaMeta is called, which is every 5s IIRC. This means if you press next right after setting the playback speed it will not be saved.

  This is a nitpick, but why not save playback speed when it is set ? Then no need to wait for saveMediaMeta, or even save it in saveMediaMeta