overhaul the entire UI/model separation

[atsushieno]

I didn't really care about the UI programming model for this toy app, I only mimicked some MV-whatever style, but it has become increasingly feature rich and now I feel hesitant to add more features because of lame code model. It's time to fix redesign and fix those issues.

An important question though is, should this still be based on .NET/C#? There is a bunch of limitations e.g. new GTK (3, not even 4...) does not get appropriate support, up to the framework level. Xamarin.Forms is based on Gtk2 which is less than okay. Other UI frameworks (Avalonia, Eto, Uno...) are too Windows-oriented which just smell. We need a considerable UI framework that respects Linux well.