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`.

core/src/keyboard/hotkey.rs

@@ -1,18 +0,0 @@
-use crate::keyboard::{KeyCode, Modifiers};
-
-/// Representation of a hotkey, consists on the combination of a [`KeyCode`] and [`Modifiers`].
-#[derive(Debug, Clone, Copy, PartialEq, Eq)]
-pub struct Hotkey {
-    /// The key that represents this hotkey.
-    pub key: KeyCode,
-
-    /// The list of modifiers that represents this hotkey.
-    pub modifiers: Modifiers,
-}
-
-impl Hotkey {
-    /// Creates a new [`Hotkey`] with the given [`Modifiers`] and [`KeyCode`].
-    pub fn new(modifiers: Modifiers, key: KeyCode) -> Self {
-        Self { modifiers, key }
-    }
-}