feat(widget): add dropdown widget as pick_list replacement

The Dropdown widget is based on the PickList widget from iced.

src/widget/dropdown/menu/appearance.rs

@@ -0,0 +1,38 @@
+// Copyright 2023 System76 <info@system76.com>
+// Copyright 2019 Héctor Ramón, Iced contributors
+// SPDX-License-Identifier: MPL-2.0 AND MIT
+
+//! Change the appearance of menus.
+use iced_core::{Background, BorderRadius, Color};
+
+/// The appearance of a menu.
+#[derive(Debug, Clone, Copy)]
+pub struct Appearance {
+    /// Menu text color
+    pub text_color: Color,
+    /// Menu background
+    pub background: Background,
+    /// Menu border width
+    pub border_width: f32,
+    /// Menu border radius
+    pub border_radius: BorderRadius,
+    /// Menu border color
+    pub border_color: Color,
+    /// Text color when hovered
+    pub hovered_text_color: Color,
+    /// Background when hovered
+    pub hovered_background: Background,
+    /// Text color when selected
+    pub selected_text_color: Color,
+    /// Background when selected
+    pub selected_background: Background,
+}
+
+/// The style sheet of a menu.
+pub trait StyleSheet {
+    /// The supported style of the [`StyleSheet`].
+    type Style: Default + Clone;
+
+    /// Produces the [`Appearance`] of a menu.
+    fn appearance(&self, style: &Self::Style) -> Appearance;
+}