app-menu: add a Calendars... item in the app menu

I won't hurt to have 2 ways to access this dialog and,
appearently, the current behavior happens to bother
some users.

https://bugzilla.gnome.org/show_bug.cgi?id=762705

data/ui/menus.ui

@@ -13,6 +13,12 @@
         <attribute name="accel">&lt;Primary&gt;f</attribute>
       </item>
     </section>
+    <section>
+      <item>
+        <attribute name="label" translatable="yes">_Calendars…</attribute>
+        <attribute name="action">win.show-calendars</attribute>
+      </item>
+    </section>
     <section>
       <item>
         <attribute name="label" translatable="yes">_Keyboard Shortcuts</attribute>