Init App.schedule systems when running the app

[smokku]

This PR adds App.schedule.initialize() call before switching to normal App event loop.
It initializes Local<> resources so the systems utilizing it do not crash.

Fixes #221

[cart]

This fixes the issue, but it would result in initialize getting double-called on the first run of apps using WinitPlugin.

To get parity with WinitPlugin's behavior I think we just need to swap this:

RunMode::Once => {
    app.schedule.run(&mut app.world, &mut app.resources);
}

for this:

RunMode::Once => {
    app.update();
}

(same goes for RunMode::Loop)

[smokku]

This fixes the issue, but it would result in initialize getting double-called on the first run of apps using WinitPlugin.

Is that a problem?
initialize() gets called on every frame anyway and just skips already initialized systems.

[cart]

Is that a problem?

In practice probably not. Its mainly an issue of logical consistency. As much as possible schedule runners should behave the same.

[smokku]

I've updated the PR.
It indeed fixes my crash too. :-)