Window hiding is absent/disabled

[4JX]

Applies to the upcoming 0.20 version. I cannot forever delay updating my dependencies, and the capability to properly hide windows is broken upstream for various reasons in various places.

I'm playing around with having more flexible/complete tray icon support without a GUI, but my time has been rather limited for quite some time now.

TODO:

• Remove the "Struct per effect" paradigm and the unecessary trait, use just the enum instead.
  • Use function(s) (effect_ui(ui)) on each file to define what UI should be shown, with a helper default_ui function for those that don't need anything special.
• Implement Unable to Save an effect #169

Blocking:

• egui Visible(false) doesn't work on wayland emilk/egui#4931
• send_viewport_cmd does not work for invisible viewports emilk/egui#3655
• Wayland: request_repaint ignored if minimized emilk/egui#5136
• App::tick emilk/egui#5113

The other option is to move out of eframe (or egui) entirely, but iced has the same issue for wayland. tauri might be the only option, not too keen about their "web renderer as UI" model though, I'd prefer the app to remain relatively self contained.