Merge remote-tracking branch 'origin/master' into feat/multi-window-support

# Conflicts:
#	Cargo.toml
#	core/src/window/icon.rs
#	core/src/window/id.rs
#	core/src/window/position.rs
#	core/src/window/settings.rs
#	examples/integration/src/main.rs
#	examples/integration_opengl/src/main.rs
#	glutin/src/application.rs
#	native/src/subscription.rs
#	native/src/window.rs
#	runtime/src/window/action.rs
#	src/lib.rs
#	src/window.rs
#	winit/Cargo.toml
#	winit/src/application.rs
#	winit/src/icon.rs
#	winit/src/settings.rs
#	winit/src/window.rs

core/src/window/id.rs

@@ -0,0 +1,28 @@
+use std::collections::hash_map::DefaultHasher;
+use std::fmt::{Display, Formatter};
+use std::hash::{Hash, Hasher};
+
+#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash, PartialOrd, Ord)]
+/// The ID of the window.
+///
+/// Internally Iced uses `window::Id::MAIN` as the first window spawned.
+pub struct Id(u64);
+
+impl Id {
+    /// The reserved window ID for the primary window in an Iced application.
+    pub const MAIN: Self = Id(0);
+
+    /// Creates a new unique window ID.
+    pub fn new(id: impl Hash) -> Id {
+        let mut hasher = DefaultHasher::new();
+        id.hash(&mut hasher);
+
+        Id(hasher.finish())
+    }
+}
+
+impl Display for Id {
+    fn fmt(&self, f: &mut Formatter<'_>) -> std::fmt::Result {
+        write!(f, "Id({})", self.0)
+    }
+}