Add richer API for describing global shortcut accelerators.

examples/shortcut.rs

@@ -8,7 +8,7 @@ fn main() {
 
 fn app(cx: Scope) -> Element {
     let toggled = use_state(cx, || false);
-    use_global_shortcut(cx, KeyCode::S, ModifiersState::CONTROL, {
+    use_global_shortcut(cx, "ctrl+s", {
         to_owned![toggled];
         move || toggled.modify(|t| !*t)
     });