Reverts flutter/flutter#141484
Initiated by: eliasyishak
This change reverts the following previous change:
Original Description:
The existing `runApp` bootstraps the widget tree and renders the provided widget into the default view (which is currently the implicit View from `PlatformDispatcher.implicitView` and - in the future - may be a default-created window). Apps, that want more control over the View they are rendered in, need a new way to bootstrap the widget tree: `runWidget`. It does not make any assumptions about the View the provided widget is rendered into. Instead, it is up to the caller to include a View widget in the provided widget tree that specifies where content should be rendered. In the future, this may enable developers to create a custom window for their app instead of relying on the default-created one.