Add Menu functionality (#65)

* Added core::Channel and menu functionality. core::Channel may leak memory.

* Added leptos example.

* Updated Channel to stream over message. Added Menu::with_items.

* Updated examples to v2 using Leptos.

* Added Menu::with_id_and_items.

* Derived Clone for drag drop events.

src/window.rs

@@ -95,15 +95,15 @@ pub(crate) struct WindowLabel {
     label: String,
 }
 
-#[derive(Deserialize, Debug)]
+#[derive(Deserialize, Clone, Debug)]
 pub enum DragDropEvent {
     Enter(DragDropPayload),
     Over(DragOverPayload),
     Drop(DragDropPayload),
     Leave,
 }
 
-#[derive(Deserialize, Debug)]
+#[derive(Deserialize, Clone, Debug)]
 pub struct DragDropPayload {
     paths: Vec<PathBuf>,
     position: dpi::PhysicalPosition,
@@ -119,7 +119,7 @@ impl DragDropPayload {
     }
 }
 
-#[derive(Deserialize, Debug)]
+#[derive(Deserialize, Clone, Debug)]
 pub struct DragOverPayload {
     position: dpi::PhysicalPosition,
 }