Recomended way to handle async splash pages with flow_builder

[aytunch]

Is your feature request related to a problem? Please describe.
Embedding optional Splash page in Flow Builder. Since splash pages are the topmost UI element, they should be taken in to consideration by this package imho.

Describe the solution you'd like
Splash pages are the first pages we see in an app. In the app initializing period, we usually make network calls and other async startup business. During this time the splash page should be ideally shown. And according to the results of the initializing functions, the flow_builder will choose the correct path.