Use existing application instance if it exists

[cmeeren]

Fixes #41

Turned out to be a devilishly simple fix. Simply use the existing application instance if it exists. AFAIK this is non-breaking, since there can only be one Application instance per AppDomain, so existing users have not been able to instantiate their own Application instance until now, ensuring that this is an opt-in feature.

With this simple change, users that need application-wide resources or otherwise need to specify an application instance can simply instantiate it before calling the Program functions. For example:

[<EntryPoint;STAThread>]
let main _argv =
  App() |> ignore
  Program.mkSimple init update view
  |> Program.runWindow (MainWindow())

Something like this should probably be added to the documentation and/or a sample.

[cmeeren]

Closed by #48