Make some Memory methods public (emilk#5046)

Adding the proposed changes from @SymmetricChaos

* Closes emilk#5044
* [x] I have followed the instructions in the PR template
* [x] I ran the check script

crates/egui/src/memory.rs

@@ -750,16 +750,20 @@ impl Memory {
         self.areas().order().iter().copied()
     }
 
-    pub(crate) fn had_focus_last_frame(&self, id: Id) -> bool {
+    /// Check if the layer had focus last frame.
+    /// returns `true` if the layer had focus last frame, but not this one.
+    pub fn had_focus_last_frame(&self, id: Id) -> bool {
         self.focus().and_then(|f| f.id_previous_frame) == Some(id)
     }
 
-    /// True if the given widget had keyboard focus last frame, but not this one.
+    /// Check if the layer lost focus last frame
+    /// returns `true` if the layer lost focus last frame, but not this one.
     pub(crate) fn lost_focus(&self, id: Id) -> bool {
         self.had_focus_last_frame(id) && !self.has_focus(id)
     }
 
-    /// True if the given widget has keyboard focus this frame, but didn't last frame.
+    /// Check if the layer gained focus this frame
+    /// returns `true` if the layer gained focus this frame, but not last one.
     pub(crate) fn gained_focus(&self, id: Id) -> bool {
         !self.had_focus_last_frame(id) && self.has_focus(id)
     }