This fixes or at least is the first step towards #171:
- it adds `cmd/super + return` as the default keybinding to toggle
fullscreen for currently focused window.
- it adds a keybinding handler to the embedded apprt and then changes
the macOS app to handle the keybinding by toggling currently focused
window.
This is my attempt at fixing #63. It works! But:
1. The `NotificationCenter` subscription is triggered once for every
open tab. That's obviously wrong. But I'm not sure and could use some
pointers where else to put the subscription. That leads me to...
2. I'm _not_ knowledgable in Swift/AppKit/SwiftUI, so I might have put
the wrong/right things in the wrong/right places. For example: wasn't
sure what's to be handled in Swift and what's to be handled by the
core in Zig.
Would love some pointers :)
