feat: app.with_on_start() function

[l0uisgrange]

A new function .with_on_start() is introduced on Xilem::new to run a FnOnce after the app has started.

let app = Xilem::new_simple(state, app_logic, opts)
  .with_on_start(move |_| {
      #[cfg(target_os = "macos")]
      menu.init_for_nsapp();
  })
app.run_in(EventLoop::with_user_event())?;

This has a few use cases, and amongst them:

1. Check for update on startup (with some API/GitHub releases/etc.)

2. More specific for macOS, but this allows compatibility with the muda crate. Without this, it's almost impossible to add items in the macOS menu bar (used by all applications), because NSApplication needs to be set before using muda functions, which is only possible after the initialization of Xilem

   If you're not familiar with macOS, the menu bar looks like this

   

[l0uisgrange]

I do not need the MasonryState argument for the use cases I introduced, but maybe someone needs it one day? I can remove it if you think it's unnecessary.

[RagibHasin]

Have you tried https://github.com/baltobor/xilem_extras ? It claims to provide application menus using muda. Although I have not personally tried it, yet.

[l0uisgrange]

Yes, I've found the library in a comment a few days ago and tried it. It is good, indeed, though I would love to bring some of its components to xilem directly.
I could not try the application menu since xilem_extras only provides a clean way to declare it, but no way to activate it (after NSApplication instantiation). Even xilem_extras would need a .with_on_start() function.