Closes#502#499
## Added
- Add `visible` method to `Panel` trait, if return false this panel will
disappear.
## Break changes
- Renamed `closeable` to `closable`.
- The `title_style`, `closable`, `zoomable` in `Panel` trait have been
change `cx: &WindowContext` to `cx: &AppContext`.
## Break Changes
- Renamed `DockItemState` to `PanelState`.
- Renamed `DockItemInfo` to `PanelInfo`.
- Update `cx` type from `WindowContext` to `AppContext` for Panel trait
`panel_name` method.
## TODO
- [ ] Support merge panels into TabPanel.
- [ ] Support split panels into Tile.
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Co-authored-by: Jason Lee <huacnlee@gmail.com>