Added a sinewave audio test source

- Added a test audio sinewave test source that should just play a sine
   wave of the middle C note.  Using unsigned 8 bit mono to test
   ffmpeg's audio resampler, seems to work pretty good.

 - Fixed a boolean trap in threading.h for the event_init function, it
   now uses enum event_type, which can be EVENT_TYPE_MANUAL or
   EVENT_TYPE_AUTO, to specify whether the event is automatically reset
   or not.

 - Changed display names of test sources to something a little less
   vague.

 - Removed te whole "if timestamp is 0 just use current system time"
   when outputting source audio, if you want to use system time you
   should just use system time yourself.  Using 0 as some sort of
   "indicator" like that just makes things confusing, and prevents you
   from legitimately using 0 as a timestamp for your audio data.

libobs/util/threading.h

@@ -53,9 +53,14 @@ struct event_data {
 	bool            manual;
 };
 
+enum event_type {
+	EVENT_TYPE_AUTO,
+	EVENT_TYPE_MANUAL
+};
+
 typedef struct event_data event_t;
 
-static inline int event_init(event_t *event, bool manual)
+static inline int event_init(event_t *event, enum event_type type)
 {
 	int code = 0;
 
@@ -65,7 +70,7 @@ static inline int event_init(event_t *event, bool manual)
 	if ((code = pthread_cond_init(&event->cond, NULL)) < 0)
 		pthread_mutex_destroy(&event->mutex);
 
-	event->manual = manual;
+	event->manual = (type == EVENT_TYPE_MANUAL);
 	event->signalled = false;
 
 	return code;