Port 'Contex Menu' demo to Vala (#34)

* Port 'Contex Menu' demo to Vala

* Improve formatting

demos/Context Menu/main.vala

@@ -0,0 +1,35 @@
+#!/usr/bin/env -S vala workbench.vala --pkg gtk4 --pkg libadwaita-1
+
+public void main() {
+  var box_menu_parent = (Gtk.Box) workbench.builder.get_object(
+    "box_menu_parent"
+  );
+  var label_emoji = (Gtk.Label) workbench.builder.get_object(
+    "label_emoji"
+  );
+  var gesture_click = (Gtk.GestureClick) workbench.builder.get_object(
+    "gesture_click"
+  );
+  var popover_menu = (Gtk.PopoverMenu) workbench.builder.get_object(
+    "popover_menu"
+  );
+
+  gesture_click.pressed.connect((gesture, n_press, x, y) => {
+    var position = Gdk.Rectangle() { x = (int) x, y = (int) y };
+    popover_menu.set_pointing_to(position);
+    popover_menu.popup();
+  });
+
+  var mood_group = new SimpleActionGroup();
+  box_menu_parent.insert_action_group("mood", mood_group);
+
+  var emoji_action = new SimpleAction(
+    "emoji",
+    new GLib.VariantType("s")
+  );
+
+  emoji_action.activate.connect((action, parameter) => {
+    label_emoji.label = parameter.get_string();
+  });
+  mood_group.add_action(emoji_action);
+}