feat: add desktop system tray support (#120)

ui/flutter/macos/Runner/AppDelegate.swift

@@ -4,6 +4,19 @@ import FlutterMacOS
 @NSApplicationMain
 class AppDelegate: FlutterAppDelegate {
   override func applicationShouldTerminateAfterLastWindowClosed(_ sender: NSApplication) -> Bool {
+    return false
+  }
+
+  override func applicationShouldHandleReopen(_ sender: NSApplication, hasVisibleWindows flag: Bool) -> Bool {
+    if !flag {
+      for window in NSApp.windows {
+        if !window.isVisible {
+          window.setIsVisible(true)
+        }
+        window.makeKeyAndOrderFront(self)
+        NSApp.activate(ignoringOtherApps: true)
+      }
+    }
     return true
   }
 }