Make egui_plot::PlotMemory public (#3871)

This allows users to e.g. read/write the plot bounds/transform before
and after showing a `Plot`.

crates/egui_plot/src/legend.rs

@@ -226,7 +226,7 @@ impl LegendWidget {
     }
 
     // Get the name of the hovered items.
-    pub fn hovered_entry_name(&self) -> Option<String> {
+    pub fn hovered_item_name(&self) -> Option<String> {
         self.entries
             .iter()
             .find(|(_, entry)| entry.hovered)