Remove Focus in pane_grid

Since #608, the `PaneGrid` widget does not handle pane focus.

wgpu/src/widget/pane_grid.rs

@@ -11,8 +11,8 @@
 use crate::Renderer;
 
 pub use iced_native::pane_grid::{
-    Axis, Configuration, Direction, DragEvent, Focus, Node, Pane, ResizeEvent,
-    Split, State,
+    Axis, Configuration, Direction, DragEvent, Node, Pane, ResizeEvent, Split,
+    State,
 };
 
 /// A collection of panes distributed using either vertical or horizontal splits