Add screen share sound effect.

src/state/CallViewModel.ts

@@ -1288,6 +1288,22 @@ export class CallViewModel extends ViewModel {
     filter((v) => v.playSounds),
   );
 
+  /**
+   * Emits an event every time a new screenshare is started in
+   * the call.
+   */
+  public readonly newScreenShare$ = this.screenShares$.pipe(
+    map((v) => v.length),
+    scan(
+      (acc, newValue) => ({
+        value: newValue,
+        playSounds: newValue > acc.value,
+      }),
+      { value: 0, playSounds: false },
+    ),
+    filter((v) => v.playSounds),
+  );
+
   public constructor(
     // A call is permanently tied to a single Matrix room and LiveKit room
     private readonly matrixRTCSession: MatrixRTCSession,