Add PlotUi::add_item(Box<dyn PlotItem>) (#51)

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* Closes <#53>
* [x] I have followed the instructions in the PR template

We have a somewhat unique setup in which we use dioxus for state
management and egui for all of the underlying component rendering. When
passing attributes through dioxus we have to cast the incoming type
which requires that the trait objects be "object safe". `PlotItem` is
not object safe, so we can't pass it through directly. But we also
cannot pass in our own trait which has a method such as `fn
as_egui_plot_item(&self) -> impl PlotItem` because that is also not
object safe so instead we have a trait which has a method `fn
as_egui_plot_item(&self) -> Box<dyn PlotItem>`. This cannot be passed to
the existing `fn add(&self, impl PlotItem)` method because `Box<dyn
PlotItem>` does not satisfy the `impl PlotItem` argument but since the
`plot_items` are stored as `Vec<Box<dyn PlotItem>>` I've added this
method to give access to insert directly.

---------

Co-authored-by: Emil Ernerfeldt <[email protected]>

egui_plot/src/plot_ui.rs

@@ -126,6 +126,11 @@ impl PlotUi {
         self.items.push(Box::new(item));
     }
 
+    /// Add an arbitrary item.
+    pub fn add_item(&mut self, item: Box<dyn PlotItem>) {
+        self.items.push(item);
+    }
+
     /// Add a data line.
     pub fn line(&mut self, mut line: crate::Line) {
         if line.series.is_empty() {