feat: showcase alert dialog

gallery/Main.qml

@@ -4,6 +4,7 @@ import Gallery
 import DesignSystem
 import QtQuick.Layouts
 import QtQuick.Controls.Basic
+import QtQuick.Dialogs
 
 AppWindow {
     id: window
@@ -49,5 +50,18 @@ AppWindow {
       }
 
       Switch {}
+
+      AppPrimaryButton {
+        text: "Open message dialog"
+        onClicked: {
+          messageDialog.open()
+        }
+      }
+
+      MessageDialog {
+        id: messageDialog
+        buttons: MessageDialog.Ok
+        text: "The document has been modified."
+      }
     }
 }