feat!(segmented-button): improved interfaces and documentation

BREAKING CHANGE: Various type and function names have changed to reflect
themselves better in documentation. Code has been reorganized into
separate modules with a better placement in libcosmic. Most of the
functions, types, and modules now have documentation and examples.

These changes no longer require the `Model` type to define the
data/component type that it stores. The component functionality is now
optional, and it's also possible to associate many components to an item
with one component per type. This has had a side effect of simplifying a
lot of the type signatures in the implementation.

Before, to insert an item into the model, you had to define a
`SegmentedItem` and a `Component` on insert, and get back an ID for that
item. Which makes it difficult to define an item that contains only an
icon or has no components. And requires an extra insert function to
activate the item on insert.

Now, there is a flexible builder-style API for configuring
newly-inserted items in the model. So the complexity for inserting and
retrieving values from the model has decreased significantly

src/widget/segmented_button/horizontal.rs

@@ -3,8 +3,7 @@
 
 //! Implementation details for the horizontal layout of a segmented button.
 
-use super::model::Model;
-use super::selection_modes::Selectable;
+use super::model::{Model, Selectable};
 use super::style::StyleSheet;
 use super::widget::{SegmentedButton, SegmentedVariant};
 
@@ -18,10 +17,12 @@ pub type HorizontalSegmentedButton<'a, SelectionMode, Message, Renderer> =
 /// A type marker defining the horizontal variant of a [`SegmentedButton`].
 pub struct Horizontal;
 
-/// Row implementation of the [`SegmentedButton`].
+/// Horizontal implementation of the [`SegmentedButton`].
+///
+/// For details on the model, see the [`segmented_button`](super) module for more details.
 #[must_use]
-pub fn horizontal_segmented_button<SelectionMode, Component, Message, Renderer>(
-    model: &Model<SelectionMode, Component>,
+pub fn horizontal<SelectionMode: Default, Message, Renderer>(
+    model: &Model<SelectionMode>,
 ) -> SegmentedButton<Horizontal, SelectionMode, Message, Renderer>
 where
     Renderer: iced_native::Renderer
@@ -29,7 +30,7 @@ where
         + iced_native::image::Renderer
         + iced_native::svg::Renderer,
     Renderer::Theme: StyleSheet,
-    SelectionMode: Selectable,
+    Model<SelectionMode>: Selectable,
 {
     SegmentedButton::new(model)
 }
@@ -42,7 +43,8 @@ where
         + iced_native::image::Renderer
         + iced_native::svg::Renderer,
     Renderer::Theme: StyleSheet,
-    SelectionMode: Selectable,
+    Model<SelectionMode>: Selectable,
+    SelectionMode: Default,
 {
     type Renderer = Renderer;