menu: support CDATA for <command> element

menu: support CDATA for <command> element

...in order to support obmenu-generator output such as this:

  <menu id="root-menu" label="Applications">
    <item label="File Manager"><action name="Execute"><command><![CDATA[xdg-open .]]></command></action></item>
    <item label="Terminal"><action name="Execute"><command><![CDATA[xterm]]></command></action></item>
    <item label="Web Browser"><action name="Execute"><command><![CDATA[xdg-open http://]]></command></action></item>
    <item label="Run command"><action name="Execute"><command><![CDATA[gmrun]]></command></action></item>
  </menu>

References:
- https://github.com/trizen/obmenu-generator
- https://aur.archlinux.org/packages/obmenu-generator
- https://trizenx.blogspot.com/2012/02/obmenu-generator.html

Fixes: issue #972