improv: Make application::Keyboard a GTK widget

This helps with some things. Having to use `OnceCell` is mildly
annoying.

Author: ids1024

Cargo.lock

@@ -24,6 +24,7 @@ gtk = { version = "0.9", features = ["v3_22"] }
 gtk-sys = "0.10"
 hidapi = "1.2"
 libc = "0.2"
+once_cell = "1.4"
 palette = "0.5"
 serde = { version = "1.0", features = ["derive"] }
 serde_json = "1.0"