Put call on hold when transfer dialog is opened (#7669)

And take it off hold if the dialog is cancelled.

Also changes the onFinished signature of invitedialog which claimed
to return an array of strings but never did, so now it just returns
a boolean.

src/CallHandler.tsx

@@ -39,6 +39,7 @@ import { WidgetType } from "./widgets/WidgetType";
 import { SettingLevel } from "./settings/SettingLevel";
 import QuestionDialog from "./components/views/dialogs/QuestionDialog";
 import ErrorDialog from "./components/views/dialogs/ErrorDialog";
+import InviteDialog, { KIND_CALL_TRANSFER } from "./components/views/dialogs/InviteDialog";
 import WidgetStore from "./stores/WidgetStore";
 import { WidgetMessagingStore } from "./stores/widgets/WidgetMessagingStore";
 import { ElementWidgetActions } from "./stores/widgets/ElementWidgetActions";
@@ -1100,6 +1101,23 @@ export default class CallHandler extends EventEmitter {
         });
     }
 
+    /*
+     * Shows the transfer dialog for a call, signalling to the other end that
+     * a transfer is about to happen
+     */
+    public showTransferDialog(call: MatrixCall): void {
+        call.setRemoteOnHold(true);
+        const { finished } = Modal.createTrackedDialog(
+            'Transfer Call', '', InviteDialog, { kind: KIND_CALL_TRANSFER, call },
+            /*className=*/"mx_InviteDialog_transferWrapper", /*isPriority=*/false, /*isStatic=*/true,
+        );
+        finished.then((results: boolean[]) => {
+            if (results.length === 0 || results[0] === false) {
+                call.setRemoteOnHold(false);
+            }
+        });
+    }
+
     private addCallForRoom(roomId: string, call: MatrixCall, changedRooms = false): void {
         if (this.calls.has(roomId)) {
             logger.log(`Couldn't add call to room ${roomId}: already have a call for this room`);