Element Call: fix widget shown while its still loading (waitForIframeLoad=false) (#12292)

* show loading spinner also if waitForIframeLoad = false
Configure EC so it waits for the content loaded action
!WARNING This breaks compatibility with the full mesh branch.
I would like to discuss here if/when we can do that.

Signed-off-by: Timo K <toger5@hotmail.de>

* stop show loading screen on widget ready (instead of preparing)

Signed-off-by: Timo K <toger5@hotmail.de>

* wait until widget loading is over before comparing screenshots

Signed-off-by: Timo K <toger5@hotmail.de>

* fix waitForIFrame=true widgets

Signed-off-by: Timo K <toger5@hotmail.de>

* test

Signed-off-by: Timo K <toger5@hotmail.de>

* always start with loading true. + cleanup

Signed-off-by: Timo K <toger5@hotmail.de>

* simplify loading

Signed-off-by: Timo K <toger5@hotmail.de>

* update snapshots (start not in loading state for waitForIframe = true widgets)

Signed-off-by: Timo K <toger5@hotmail.de>

---------

Signed-off-by: Timo K <toger5@hotmail.de>

test/test-utils/call.ts

@@ -38,9 +38,8 @@ export class MockedCall extends Call {
                 url: "https://example.org",
                 name: "Group call",
                 creatorUserId: "@alice:example.org",
-                // waitForIframeLoad = false, makes the widget API wait for the 'contentLoaded' event instead.
-                // This is how the EC is designed, but for backwards compatibility (full mesh) we currently need to use waitForIframeLoad = true
-                // waitForIframeLoad: false
+                // waitForIframeLoad = false, makes the widget API wait for the 'contentLoaded' event.
+                waitForIframeLoad: false,
             },
             room.client,
         );