Add `Memory::move_focus` (#7476)

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This allows us to programatically move focus around. Another way would
be to inject arrow key inputs, but that requires the backend to
implement a way to do that, and could have unintended side-effects.

Co-authored-by: Emil Ernerfeldt <emil.ernerfeldt@gmail.com>

crates/egui/src/lib.rs

@@ -499,7 +499,7 @@ pub use self::{
     layers::{LayerId, Order},
     layout::*,
     load::SizeHint,
-    memory::{Memory, Options, Theme, ThemePreference},
+    memory::{FocusDirection, Memory, Options, Theme, ThemePreference},
     painter::Painter,
     response::{InnerResponse, Response},
     sense::Sense,

crates/egui/src/memory/mod.rs

@@ -140,8 +140,9 @@ impl Default for Memory {
     }
 }
 
+/// A direction in which to move the keyboard focus.
 #[derive(Clone, Copy, Debug, Default, PartialEq, Eq)]
-enum FocusDirection {
+pub enum FocusDirection {
     /// Select the widget closest above the current focused widget.
     Up,
 
@@ -888,6 +889,11 @@ impl Memory {
         }
     }
 
+    /// Move keyboard focus in a specific direction.
+    pub fn move_focus(&mut self, direction: FocusDirection) {
+        self.focus_mut().focus_direction = direction;
+    }
+
     /// Returns true if
     /// - this layer is the top-most modal layer or above it
     /// - there is no modal layer