frontend: Make builder property public

vanilla_first_setup/window.py

@@ -53,6 +53,10 @@ class VanillaWindow(Adw.ApplicationWindow):
         # connect system signals
         self.__connect_signals()
     
+    @property
+    def builder(self):
+        return self.__builder
+
     def __connect_signals(self):
         self.btn_back.connect("clicked", self.back)
         self.carousel.connect("page-changed", self.__on_page_changed)