Menu icons must be in png format with a alpha channel.

The menu icons can be localized by adding them in the same directory structure
as the default icons but with a root directory named after the langauge code
of the locale. eg For Dutch 'nl'.

The default directory tree looks like this:
 menuicons/...
 menuicons/submenu/...

So for Dutch we would have the following:
 menuicons/nl/...
 menuicons/nl/submenu/...

Beware that the menu code doesn't check for the existence of the localized
icons. You must make sure all the icons that are mentioned in the  menu xml
file are present.
