Main entry point, but have few difference from Browser.document

difference from Browser.document:

1. init takes as argument flags and returns Task, that results in world or fail (90% of games any way need get some data to start, images, level data, connection, etc.)

2. subscriptions instead of returning msg - should return new World

3. update have no Msg input

4. view instead of Msg event have to return function

Error type to hold all error of game init

just alias to Logic.GameFlow.Model world

Helper function, that allow convert Task into Cmd - for loading new World after init

Main entry point, but have few difference from Platform.worker