widget-factory: Move app menu contents to primary menu

GNOME Shell 3.32 will remove support for the app menu
so we need to move its contents to the primary (hamburger)
menu.

widget-factory already had a primary menu.

The only item in the app menu was About.

https://gitlab.gnome.org/GNOME/Initiatives/issues/4

demos/widget-factory/menus.ui

@@ -1,19 +0,0 @@
-<?xml version="1.0" encoding="UTF-8"?>
-<interface>
-  <!-- interface-requires gtk+ 3.0 -->
-  <menu id="app-menu">
-    <section>
-      <item>
-        <attribute name="label" translatable="yes">About</attribute>
-        <attribute name="action">app.about</attribute>
-      </item>
-    </section>
-    <section>
-      <item>
-        <attribute name="label" translatable="yes">_Quit</attribute>
-        <attribute name="action">app.quit</attribute>
-        <attribute name="accel">&lt;Primary&gt;q</attribute>
-      </item>
-    </section>
-  </menu>
-</interface>

demos/widget-factory/widget-factory.gresource.xml

@@ -6,9 +6,6 @@
   <gresource prefix="/org/gtk/WidgetFactory">
     <file>widget-factory.css</file>
   </gresource>
-  <gresource prefix="/org/gtk/WidgetFactory/gtk">
-    <file preprocess="xml-stripblanks">menus.ui</file>
-  </gresource>
   <gresource prefix="/org/gtk/WidgetFactory/gtk">
     <file preprocess="xml-stripblanks">help-overlay.ui</file>
   </gresource>

demos/widget-factory/widget-factory.ui

@@ -16,6 +16,12 @@
         <attribute name="action">win.transition</attribute>
       </item>
     </section>
+    <section>
+      <item>
+        <attribute name="label" translatable="yes">About</attribute>
+        <attribute name="action">app.about</attribute>
+      </item>
+    </section>
   </menu>
   <menu id="dinner_menu">
     <section>