Introduce window::Level enum

... and add `level` field to `window::Settings`

core/src/window.rs

@@ -2,12 +2,14 @@
 pub mod icon;
 
 mod event;
+mod level;
 mod mode;
 mod redraw_request;
 mod user_attention;
 
 pub use event::Event;
 pub use icon::Icon;
+pub use level::Level;
 pub use mode::Mode;
 pub use redraw_request::RedrawRequest;
 pub use user_attention::UserAttention;

core/src/window/level.rs

@@ -0,0 +1,19 @@
+/// A window level groups windows with respect to their z-position.
+///
+/// The relative ordering between windows in different window levels is fixed.
+/// The z-order of a window within the same window level may change dynamically
+/// on user interaction.
+#[derive(Debug, Clone, Copy, PartialEq, Eq, Default)]
+pub enum Level {
+    /// The default behavior.
+    #[default]
+    Normal,
+
+    /// The window will always be below normal windows.
+    ///
+    /// This is useful for a widget-based app.
+    AlwaysOnBottom,
+
+    /// The window will always be on top of normal windows.
+    AlwaysOnTop,
+}