list: Refactor ListDelegate to easy access delegate. (#1713)
Continue #1712
## Break Changes
- The argument type of `render_item`, `render_section_header`,
`render_section_footer`, `render_empty`, `render_initial`,
`render_loading` in `ListDelegate` has been changed to has `&mut self`
and `&mut Context<ListState<Self>>`.
```diff
- fn render_item(&self, ix: IndexPath, window: &mut Window, cx: &mut App) -> Option<Self::Item>
+ fn render_item(&mut self, ix: IndexPath, window: &mut Window, cx: &mut Context<ListState<Self>>) -> Option<Self::Item>
```
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