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, as it turns out, the simple version of #166.
It changes the quit dialog to handle return by removing the
`.destructive` role from the `Quit` button.
When that role is set, the `keyboardShortcut(.defaultAction)` doesn't have an effect.
With the role removed, the dialog handles the return key to quit the
application
As I wrote in #166, I can understand if you don't want this change, but
I personally think it's nice to be able to only use the keyboard. iTerm2
also handles return like this in the "Are you sure?" dialog.