Simpler APIs (#28)

* TrackSource

* remove absolute import

* typo

* get track

* set source enabled

* fix symbol collision

* move to participant

* update proto

* screen share audio

* typo

* format

* change screen share name

lib/src/types.dart

@@ -29,6 +29,14 @@ enum Reliability {
   lossy,
 }
 
+enum TrackSource {
+  unknown,
+  camera,
+  microphone,
+  screenShareVideo,
+  screenShareAudio,
+}
+
 enum CloseReason {
   network,
   // ...