Use hook_TYPE_alter (hook_menu_link_alter) for improved compatibility

Issue Summary

The attached patch modifies menu_icons to use hook_TYPE_alter to do most of the menu link option processing instead of doing it all in the menu item edit form submit function (which is the more Drupal way of doing things). This allows other modules that alter menu links to alter the menu icon options, and perhaps more importantly properly import and export menu links with menu icons (eg the Features module).

The form submit process function is modified to only handle the file upload field, everything else is done in the (new) function menu_icons_menu_link_alter.