Revert system menus support

The current implementation has some important issues on Windows. We will reintroduce the feature once we figure them out!

I have kept some of the changes in #945, like the new `keyboard::Modifiers` powered by `bitflags`.

src/application.rs

@@ -1,5 +1,5 @@
 use crate::window;
-use crate::{Color, Command, Element, Executor, Menu, Settings, Subscription};
+use crate::{Color, Command, Element, Executor, Settings, Subscription};
 
 /// An interactive cross-platform application.
 ///
@@ -97,7 +97,7 @@ pub trait Application: Sized {
     type Executor: Executor;
 
     /// The type of __messages__ your [`Application`] will produce.
-    type Message: std::fmt::Debug + Clone + Send;
+    type Message: std::fmt::Debug + Send;
 
     /// The data needed to initialize your [`Application`].
     type Flags;
@@ -185,13 +185,6 @@ pub trait Application: Sized {
         false
     }
 
-    /// Returns the current system [`Menu`] of the [`Application`].
-    ///
-    /// By default, it returns an empty [`Menu`].
-    fn menu(&self) -> Menu<Self::Message> {
-        Menu::new()
-    }
-
     /// Runs the [`Application`].
     ///
     /// On native platforms, this method will take control of the current thread
@@ -293,10 +286,6 @@ where
     fn should_exit(&self) -> bool {
         self.0.should_exit()
     }
-
-    fn menu(&self) -> Menu<Self::Message> {
-        self.0.menu()
-    }
 }
 
 #[cfg(target_arch = "wasm32")]

src/lib.rs

@@ -246,7 +246,6 @@ pub use sandbox::Sandbox;
 pub use settings::Settings;
 
 pub use runtime::{
-    futures, menu, Align, Background, Color, Command, Font,
-    HorizontalAlignment, Length, Menu, Point, Rectangle, Size, Subscription,
-    Vector, VerticalAlignment,
+    futures, Align, Background, Color, Command, Font, HorizontalAlignment,
+    Length, Point, Rectangle, Size, Subscription, Vector, VerticalAlignment,
 };

src/sandbox.rs

@@ -87,7 +87,7 @@ use crate::{
 /// ```
 pub trait Sandbox {
     /// The type of __messages__ your [`Sandbox`] will produce.
-    type Message: std::fmt::Debug + Clone + Send;
+    type Message: std::fmt::Debug + Send;
 
     /// Initializes the [`Sandbox`].
     ///