Make the hangup button do things for conference calls

Behaviour constraints:
* If you're not in the conference, use a grey button that does nothing.
* If you're in the conference, show a button:
  * If you're able to modify widgets in the room, annotate it in the context of ending the call for everyone and remove the widget. Use a confirmation dialog.
  * If you're not able to modify widgets in the room, hang up.

For this we know that persistent Jitsi widgets will mean that the user is in the call, so we use that to determine if they are actually participating.

src/widgets/WidgetApi.ts

@@ -39,6 +39,7 @@ export enum KnownWidgetActions {
     SetAlwaysOnScreen = "set_always_on_screen",
     ClientReady = "im.vector.ready",
     Terminate = "im.vector.terminate",
+    Hangup = "im.vector.hangup",
 }
 
 export type WidgetAction = KnownWidgetActions | string;
@@ -119,13 +120,15 @@ export class WidgetApi extends EventEmitter {
 
                     // Automatically acknowledge so we can move on
                     this.replyToRequest(<ToWidgetRequest>payload, {});
-                } else if (payload.action === KnownWidgetActions.Terminate) {
+                } else if (payload.action === KnownWidgetActions.Terminate
+                    || payload.action === KnownWidgetActions.Hangup) {
                     // Finalization needs to be async, so postpone with a promise
                     let finalizePromise = Promise.resolve();
                     const wait = (promise) => {
                         finalizePromise = finalizePromise.then(() => promise);
                     };
-                    this.emit('terminate', wait);
+                    const emitName = payload.action === KnownWidgetActions.Terminate ? 'terminate' : 'hangup';
+                    this.emit(emitName, wait);
                     Promise.resolve(finalizePromise).then(() => {
                         // Acknowledge that we're shut down now
                         this.replyToRequest(<ToWidgetRequest>payload, {});