split property parsing from event parsing:

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.

src/core_api.rs

@@ -75,6 +75,7 @@ pub enum MpvDataType {
     Double(f64),
     HashMap(HashMap<String, MpvDataType>),
     Null,
+    MinusOne,
     Playlist(Playlist),
     String(String),
     Usize(usize),