Make Zenity backend optional

[Ralith]

Saves some Rust dependencies for users who are willing to require gtk3 (the default) or who don't want message dialogs at all.

[Ralith]

Alternatively, the zenity backend could be rewritten to spawn a background thread rather than relying on async-io, making it much lighter weight and more portable. Adaptation to futures could use something like oneshot or a trivial hand-rolled channel.

[PolyMeilex]

Alternatively, the zenity backend could be rewritten to spawn a background thread

Sounds like a better idea, I will do that instead in spare time.

[Ralith]

Thanks!