Added menu functionality. (#58)

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

* Added leptos example.

examples/leptos/src-tauri/capabilities/default.json

@@ -0,0 +1,17 @@
+{
+  "$schema": "../gen/schemas/desktop-schema.json",
+  "identifier": "default",
+  "description": "Capability for the main window",
+  "windows": ["main"],
+  "permissions": [
+    "core:path:default",
+    "core:event:default",
+    "core:window:default",
+    "core:app:default",
+    "core:image:default",
+    "core:resources:default",
+    "core:menu:default",
+    "core:tray:default",
+    "shell:allow-open"
+  ]
+}