add custom application to redirect lifecycle calls to event handler

[stroborobo]

Hey Fabulous team,

added a custom application class so we can have an event handler for the application lifecycle methods. It's clearly inspired by the custom picker and others, not sure if this is the intended way, appears to be working for me. Feedback very welcome if I should do it differently.

Also, are the docs generated or should I put it in there manually?

[stroborobo]

fixed formatting

[TimLariviere]

Hi. I'm currently on holidays. I'll review your PR in more details later. But first glance, the code seems good 👌

For the documentation, it's manual.

Also do you have a quick explanation for your use case with those events?

[stroborobo]

I'm periodically fetching updated data while the app is open, so I'm both fetching updates and starting a subscription on resume and cancel it's cancellationtoken on sleep.