Add a public API for overriding plot legend traces' visibilities (#3534)

Added a public API in `egui_plot -> legend` to allow `hidden_items` to
be overridden in the plot legend widget. This allows convenient control
of traces' visibilities the selection of traces from the application
code.

### Example
```rust
    let legend_config = if plot_selection_changed {
        let hidden_items = match plot_selection {
            PlotSelection::SelectAll => Vec::new(),
            PlotSelection::DeselectAll => all_trace_names,
        };

        Legend::default()
            .position(Corner::RightTop)
            .hidden_items(hidden_items) // Overrides `hidden_items`
    } else {
        Legend::default().position(Corner::RightTop)
    };

    Plot::new(id)
        .legend(legend_config)
        .show(ui, draw_plot);
```

Closes <https://github.com/emilk/egui/issues/3533>.

---------

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

crates/egui_plot/src/legend.rs

@@ -32,6 +32,9 @@ pub struct Legend {
     pub text_style: TextStyle,
     pub background_alpha: f32,
     pub position: Corner,
+
+    /// Used for overriding the `hidden_items` set in [`LegendWidget`].
+    hidden_items: Option<ahash::HashSet<String>>,
 }
 
 impl Default for Legend {
@@ -40,6 +43,8 @@ impl Default for Legend {
             text_style: TextStyle::Body,
             background_alpha: 0.75,
             position: Corner::RightTop,
+
+            hidden_items: None,
         }
     }
 }
@@ -65,6 +70,17 @@ impl Legend {
         self.position = corner;
         self
     }
+
+    /// Specifies hidden items in the legend configuration to override the existing ones. This
+    /// allows the legend traces' visibility to be controlled from the application code.
+    #[inline]
+    pub fn hidden_items<I>(mut self, hidden_items: I) -> Self
+    where
+        I: IntoIterator<Item = String>,
+    {
+        self.hidden_items = Some(hidden_items.into_iter().collect());
+        self
+    }
 }
 
 #[derive(Clone)]
@@ -167,8 +183,11 @@ impl LegendWidget {
         rect: Rect,
         config: Legend,
         items: &[Box<dyn PlotItem>],
-        hidden_items: &ahash::HashSet<String>,
+        hidden_items: &ahash::HashSet<String>, // Existing hiddent items in the plot memory.
     ) -> Option<Self> {
+        // If `config.hidden_items` is not `None`, it is used.
+        let hidden_items = config.hidden_items.as_ref().unwrap_or(hidden_items);
+
         // Collect the legend entries. If multiple items have the same name, they share a
         // checkbox. If their colors don't match, we pick a neutral color for the checkbox.
         let mut entries: BTreeMap<String, LegendEntry> = BTreeMap::new();