Add menubar example

Not working yet, but at least it shows how the API should be used.

examples/menubar.rs

@@ -0,0 +1,24 @@
+extern crate cursive;
+
+use cursive::Cursive;
+use cursive::view::Dialog;
+use cursive::event::Key;
+
+fn main() {
+
+    let mut siv = Cursive::new();
+
+    siv.menu()
+       .new_subtree("File")
+       .leaf("New", |s| s.add_layer(Dialog::info("New file!")))
+       .leaf("Quit", |s| s.quit());
+
+    siv.menu()
+       .new_subtree("Help")
+       .leaf("Help", |s| s.add_layer(Dialog::info("Help message!")))
+       .leaf("About", |s| s.add_layer(Dialog::info("Cursive v0.0.0")));
+
+    siv.add_global_callback(Key::F(10), |s| s.select_menu());
+
+    siv.run();
+}

src/menu.rs

@@ -48,10 +48,10 @@ impl MenuTree {
         self
     }
 
-    pub fn new_subtree(&mut self, title: &str) -> &mut Box<MenuTree> {
+    pub fn new_subtree(&mut self, title: &str) -> &mut MenuTree {
         self.add_subtree(title, MenuTree::new());
         match *self.children.last_mut().unwrap() {
-            MenuItem::Subtree(_, ref mut tree) => tree,
+            MenuItem::Subtree(_, ref mut tree) => &mut *tree,
             _ => panic!("??"),
         }
     }