Added menu functionality. (#58)

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

* Added leptos example.

examples/leptos/src-tauri/tauri.conf.json

@@ -0,0 +1,35 @@
+{
+  "productName": "test-tauri-events",
+  "version": "0.0.0",
+  "identifier": "com.tauri.dev",
+  "build": {
+    "beforeDevCommand": "trunk serve",
+    "devUrl": "http://localhost:1420",
+    "beforeBuildCommand": "trunk build",
+    "frontendDist": "../dist"
+  },
+  "app": {
+    "withGlobalTauri": true,
+    "windows": [
+      {
+        "title": "test-tauri-events",
+        "width": 800,
+        "height": 600
+      }
+    ],
+    "security": {
+      "csp": null
+    }
+  },
+  "bundle": {
+    "active": true,
+    "targets": "all",
+    "icon": [
+      "icons/32x32.png",
+      "icons/128x128.png",
+      "icons/128x128@2x.png",
+      "icons/icon.icns",
+      "icons/icon.ico"
+    ]
+  }
+}