Fix embedded Element Call screen sharing (#9485)

* Fix embedded Element Call screen sharing

Makes it a request in each direction rather than a request and reply
since replies to requests time out and so can't wait for user interaction.

* Fix tests

src/stores/widgets/ElementWidgetActions.ts

@@ -26,10 +26,23 @@ export enum ElementWidgetActions {
     MuteVideo = "io.element.mute_video",
     UnmuteVideo = "io.element.unmute_video",
     StartLiveStream = "im.vector.start_live_stream",
+
+    // Element Call -> host requesting to start a screenshare
+    // (ie. expects a ScreenshareStart once the user has picked a source)
+    // replies with { pending } where pending is true if the host has asked
+    // the user to choose a window and false if not (ie. if the host isn't
+    // running within Electron)
+    ScreenshareRequest = "io.element.screenshare_request",
+    // host -> Element Call telling EC to start screen sharing with
+    // the given source
+    ScreenshareStart = "io.element.screenshare_start",
+    // host -> Element Call telling EC to stop screen sharing, or that
+    // the user cancelled when selecting a source after a ScreenshareRequest
+    ScreenshareStop = "io.element.screenshare_stop",
+
     // Actions for switching layouts
     TileLayout = "io.element.tile_layout",
     SpotlightLayout = "io.element.spotlight_layout",
-    Screenshare = "io.element.screenshare",
 
     OpenIntegrationManager = "integration_manager_open",