Prepare for Element Call integration (#9224)

* Improve accessibility and testability of Tooltip

Adding a role to Tooltip was motivated by React Testing Library's
reliance on accessibility-related attributes to locate elements.

* Make the ReadyWatchingStore constructor safer

The ReadyWatchingStore constructor previously had a chance to
immediately call onReady, which was dangerous because it was potentially
calling the derived class's onReady at a point when the derived class
hadn't even finished construction yet. In normal usage, I guess this
never was a problem, but it was causing some of the tests I was writing
to crash. This is solved by separating out the onReady call into a start
method.

* Rename 1:1 call components to 'LegacyCall'

to reflect the fact that they're slated for removal, and to not clash
with the new Call code.

* Refactor VideoChannelStore into Call and CallStore

Call is an abstract class that currently only has a Jitsi
implementation, but this will make it easy to later add an Element Call
implementation.

* Remove WidgetReady, ClientReady, and ForceHangupCall hacks

These are no longer used by the new Jitsi call implementation, and can
be removed.

* yarn i18n

* Delete call map entries instead of inserting nulls

* Allow multiple active calls and consolidate call listeners

* Fix a race condition when creating a video room

* Un-hardcode the media device fallback labels

* Apply misc code review fixes

* yarn i18n

* Disconnect from calls more politely on logout

* Fix some strict mode errors

* Fix another updateRoom race condition

src/stores/WidgetStore.ts

@@ -36,7 +36,7 @@ export interface IApp extends IWidget {
     roomId: string;
     eventId: string;
     // eslint-disable-next-line camelcase
-    avatar_url: string; // MSC2765 https://github.com/matrix-org/matrix-doc/pull/2765
+    avatar_url?: string; // MSC2765 https://github.com/matrix-org/matrix-doc/pull/2765
 }
 
 interface IRoomWidgets {
@@ -46,7 +46,11 @@ interface IRoomWidgets {
 // TODO consolidate WidgetEchoStore into this
 // TODO consolidate ActiveWidgetStore into this
 export default class WidgetStore extends AsyncStoreWithClient<IState> {
-    private static internalInstance = new WidgetStore();
+    private static readonly internalInstance = (() => {
+        const instance = new WidgetStore();
+        instance.start();
+        return instance;
+    })();
 
     private widgetMap = new Map<string, IApp>(); // Key is widget Unique ID (UID)
     private roomMap = new Map<string, IRoomWidgets>(); // Key is room ID