Application lifecycle hooks

[vallemar]

I think it would be nice to have a specific page about the application lifecycle, also the android and iOS part https://v7.docs.nativescript.org/core-concepts/application-lifecycle#use-application-events

[vallemar]

For more documentation, I have recently learned on discord that there are more events that are not documented in the documentation above. All the events there are: https://github.com/NativeScript/NativeScript/blob/main/packages/core/application/application-common.ts#L25

[Ombuweb]

@vallemar The application lifecycle events are documented in this PR.