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/scrollable.rs

@@ -1,8 +1,8 @@
 #[macro_export]
 macro_rules! scrollable {
-    ($x:expr) => (
+    ($x:expr) => {
         $crate::iced::widget::scrollable($x)
             .scrollbar_width(8)
             .scroller_width(8)
-    );
-}
+    };
+}