improv(segmented-button): Express vertical/horizontal variants as a state machine

It's difficult to make iterative developments when there's two
nearly-identical types that need to be kept synchronized to any change.
Rust gives us traits so we should use them instead of duplicating code.

This made it easier to make styling and layout improvements to both
instances of the segmented button.

src/widget/segmented_button/style.rs

@@ -4,25 +4,35 @@
 use iced_core::{Background, BorderRadius, Color};
 
 /// The appearance of a segmented button.
-#[derive(Clone, Copy)]
+#[derive(Default, Clone, Copy)]
 pub struct Appearance {
     pub background: Option<Background>,
-    pub border_color: Color,
     pub border_radius: BorderRadius,
-    pub border_width: f32,
-    pub button_active: ButtonAppearance,
-    pub button_inactive: ButtonAppearance,
-    pub button_hover: ButtonAppearance,
+    pub border_bottom: Option<(f32, Color)>,
+    pub border_end: Option<(f32, Color)>,
+    pub border_start: Option<(f32, Color)>,
+    pub border_top: Option<(f32, Color)>,
+    pub active: ButtonStatusAppearance,
+    pub inactive: ButtonStatusAppearance,
+    pub hover: ButtonStatusAppearance,
 }
 
 /// The appearance of a button in the segmented button
-#[derive(Clone, Copy)]
+#[derive(Default, Clone, Copy)]
 pub struct ButtonAppearance {
-    pub background: Option<Background>,
+    pub border_radius: BorderRadius,
     pub border_bottom: Option<(f32, Color)>,
-    pub border_radius_first: BorderRadius,
-    pub border_radius_middle: BorderRadius,
-    pub border_radius_last: BorderRadius,
+    pub border_end: Option<(f32, Color)>,
+    pub border_start: Option<(f32, Color)>,
+    pub border_top: Option<(f32, Color)>,
+}
+
+#[derive(Default, Clone, Copy)]
+pub struct ButtonStatusAppearance {
+    pub background: Option<Background>,
+    pub first: ButtonAppearance,
+    pub middle: ButtonAppearance,
+    pub last: ButtonAppearance,
     pub text_color: Color,
 }