This would allow implementing a way to convert between different scales and
which in turn would allow the UI to provide an editor with a generic scale
(e.g. stars) instead of only allowing to edit raw values as string.
This also make it assume that a single number is meant to be the rating
(instead of the user). That should make editing the rating a bit more
straight forward (if one doesn't care about the user and play counter).
* Propose the usage of the popularity type in general for the rating field
so GUIs can show an appropriate UI element
* Do not just propose the popularity type for ID3v2 tags so a uniform UI
element can be shown accross tag formats; and API to convert from a
uniform scale is still TODO
That an automatic conversion happens for different types but not
for different encodings was always a bit odd.
This makes writing tests easier and comparing values within the
tag editor does not rely on choosing a particular encoding.
