data: Add application menu ui file.

<?xml version="1.0"?>

<interface>

  <requires lib="gtk+" version="3.12"/>

  <menu id="appmenu">

    <section>

      <item>

        <attribute name="label" translatable="yes">_Preferences</attribute>

        <attribute name="action">app.preferences</attribute>

      </item>

    </section>

    <section>

      <item>

        <attribute name="label" translatable="yes">_Quit</attribute>

        <attribute name="action">app.quit</attribute>

      </item>

    </section>

  </menu>

</interface>