Events: Add Play/Pause events.

Uses much of the existing machinery to add some more PlayMode state change events—all in all, a pretty simple change.

Closes #29.

src/events/track.rs

@@ -10,6 +10,14 @@
 #[derive(Clone, Copy, Debug, Eq, Hash, PartialEq)]
 #[non_exhaustive]
 pub enum TrackEvent {
+    /// The attached track has resumed playing.
+    ///
+    /// This event will not fire when a track first starts,
+    /// but will fire when a track changes from, e.g., paused to playing.
+    /// This is most relevant for queue users.
+    Play,
+    /// The attached track has been paused.
+    Pause,
     /// The attached track has ended.
     End,
     /// The attached track has looped.