improv(menu): simplify menu construction.

- Added `MenuAction` trait to call the `message` method on button press.
- Added two new methods to construct a MenuTree.
- Added MenuItem enum to represent an action or a separator in a MenuTree.
- Added menu example.
- Moved Modifier enum and KeyBind struct to libcosmic.
- Moved menu_button macro to libcosmic.

src/widget/menu/key_bind.rs

@@ -0,0 +1,39 @@
+use iced_core::keyboard::{Key, Modifiers};
+use std::fmt;
+
+#[derive(Clone, Copy, Debug, Eq, Hash, Ord, PartialEq, PartialOrd)]
+pub enum Modifier {
+    Super,
+    Ctrl,
+    Alt,
+    Shift,
+}
+
+#[derive(Clone, Debug, Eq, Hash, Ord, PartialEq, PartialOrd)]
+pub struct KeyBind {
+    pub modifiers: Vec<Modifier>,
+    pub key: Key,
+}
+
+impl KeyBind {
+    pub fn matches(&self, modifiers: Modifiers, key: &Key) -> bool {
+        key == &self.key
+            && modifiers.logo() == self.modifiers.contains(&Modifier::Super)
+            && modifiers.control() == self.modifiers.contains(&Modifier::Ctrl)
+            && modifiers.alt() == self.modifiers.contains(&Modifier::Alt)
+            && modifiers.shift() == self.modifiers.contains(&Modifier::Shift)
+    }
+}
+
+impl fmt::Display for KeyBind {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        for modifier in self.modifiers.iter() {
+            write!(f, "{:?} + ", modifier)?;
+        }
+        match &self.key {
+            Key::Character(c) => write!(f, "{}", c.to_uppercase()),
+            Key::Named(named) => write!(f, "{:?}", named),
+            other => write!(f, "{:?}", other),
+        }
+    }
+}