dock: Refactor Panel trait to has &mut self and &mut Context<Self>. (#1716)
Continue #1712, #1713
## Description
Also change the `Panel` trait to has `&mut self` and `&mut
Context<Self>`.
## Break Changes
The methods `title`, `title_prefix`, `set_zoomed`, `set_active`,
`dropdown_menu`, `toolbar_buttons`, `on_added_to`, `on_removed` has
changed `&self` to `&mut self`, and `cx: &App` to `cx: &mut
Context<Self>`.
```diff
- fn title(&self, window: &Window, cx: &App) -> AnyElement
+ fn title(&mut self, window: &Window, cx: &mut Context<Self>) -> AnyElement
```
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