Send a 'close' action when the widget is ready to close

By keeping 'hangup' and 'close' as separate actions, we can allow Element Call widgets to stay on an error screen after the user has been disconnected without the widget completely disappearing from the host's UI. We don't have to request any additional capabilities to use a custom widget action like this one.

src/widget.ts

@@ -21,10 +21,11 @@ import { getUrlParams } from "./UrlParams";
 import { Config } from "./config/Config";
 import { ElementCallReactionEventType } from "./reactions";
 
-// Subset of the actions in matrix-react-sdk
+// Subset of the actions in element-web
 export enum ElementWidgetActions {
   JoinCall = "io.element.join",
   HangupCall = "im.vector.hangup",
+  Close = "io.element.close",
   TileLayout = "io.element.tile_layout",
   SpotlightLayout = "io.element.spotlight_layout",
   // This can be sent as from or to widget