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/mod.rs

@@ -25,7 +25,6 @@ pub use toggler::toggler;
 pub mod segmented_button;
 pub use segmented_button::{
     horizontal_segmented_button, vertical_segmented_button, HorizontalSegmentedButton,
-    VerticalSegmentedButton,
 };
 
 pub mod settings;