Add _xdg_list_swap method.
Add public methods for manipulation of XdgList objects:
 - xdg_list_swap;
 - xdg_list_remove.
New features and bug fixes:
 - xdg_user_defined_apps_load;
 - xdg_user_defined_apps_lookup;
 - xdg_user_defined_apps_head;
 - xdg_user_defined_apps_append;
 - xdg_user_defined_apps_prepend;
 - xdg_user_defined_apps_free.