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/mod.rs

@@ -24,3 +24,6 @@ pub use scrollable::*;
 
 mod expander;
 pub use expander::*;
+
+pub mod list_box;
+pub use list_box::*;