Enable screen sharing

For now, only the video is transmitted.
The camera is turned off when screensharing, just like on the desktop
app.

In CallProvider, replace `isVideoOn` state with `videoStatus` to toggle
between off, on or screenshare.
Add icon for when screenshare is active.
Remove screenshare button menu options, as they are unavailable on the
web.

Change-Id: I95dd1c18efd91ab8b9063f0e7f817839e4f24aba
diff --git a/client/src/utils/utils.ts b/client/src/utils/utils.ts
index 770c7a8..d51fa1b 100644
--- a/client/src/utils/utils.ts
+++ b/client/src/utils/utils.ts
@@ -21,7 +21,9 @@
   children: ReactNode;
 };
 
+type AsyncDispatch<A> = (value: A) => Promise<void>;
 export type SetState<T> = Dispatch<SetStateAction<T>>;
+export type AsyncSetState<T> = AsyncDispatch<SetStateAction<T>>;
 
 /**
  * HTMLVideoElement with the `sinkId` and `setSinkId` optional properties.