Add 'update_properties' signal and obs_source_update_properties call

obs_source_update_properties should be called by sources when property
values change, e.g. a capture device source would use this when it
detects a new capture device (in case its properties contain a list of
available capture devices or similar)

libobs/obs.h

@@ -730,6 +730,9 @@ EXPORT void obs_source_output_video(obs_source_t *source,
 EXPORT void obs_source_output_audio(obs_source_t *source,
 		const struct obs_source_audio *audio);
 
+/** Signal an update to any currently used properties via 'update_properties' */
+EXPORT void obs_source_update_properties(obs_source_t *source);
+
 /** Gets the current async video frame */
 EXPORT struct obs_source_frame *obs_source_get_frame(obs_source_t *source);