Implemented Expander

- Updated example to show behavior
- Created styles for Expander and ExpanderRow
- Simpler implementation of `ExpanderRow`
- Deleted `ExpanderData` and replaced it with `ExpanderRow`
- Every row can now have child rows.
- Ran cargo fmt.
- Deleted settings example
- Added expander to cosmic example
- Expander icons now render

ListBox partially implemented

src/widget/toggler.rs

@@ -1,7 +1,4 @@
-use iced::{
-    Length,
-    widget,
-};
+use iced::{widget, Length};
 
 pub fn toggler<'a, Message, Renderer>(
     label: impl Into<Option<String>>,