client API: make "cache" property and similar observable

Achieve this by polling. Will be used by the OSC. Basically a bad hack -
but the point is that the mpv core itself is in the best position to
improve this later.

libmpv/client.h

@@ -983,6 +983,7 @@ typedef enum mpv_event_id {
      * Happens when the current chapter changes.
      */
     MPV_EVENT_CHAPTER_CHANGE = 23
+    // Internal note: adjust INTERNAL_EVENT_BASE when adding new events.
 } mpv_event_id;
 
 /**