feat(designer): Make Designer compilable for other platforms than web

[johannesvedder]

At the moment the Designer can only run on the web platform because of the html and js libraries used for the test platform. If we add alternative implementations for this, the Designer could also be run with native dart on Windows/Linux/Mac systems. I found two options that could be used for a native dart implementation of the test screen to embedd the App.

• The webview_flutter package could help to embed the App website. However, we probably lose instant preview feature, because there is probably no communication possible between the Designer and the webview.

• We might be able to use MaterialApp() inside the Designer to embed the App. This will probably also need changes in the App to support communication between the two apps. We need to test whether and how big the impact on performance is.