This brings several changes with it:
- `Mpv::get_property` now returns `Option`s in case `data` is
nonexistent. There could be cases where this is different from
`MpvDataType::Null` and `MpvDataType::MinusOne`.
- `MpvError` now implements `PartialEq`
High-level properties are now optional, considering there are about a
thousand of them to parse. The high-level properties are a few chosen
ones that I suspect might be useful for most people, with catch-all enum
variants for the less common ones.