Record 'option_order' when passed through xprofile_insert_field()

Previously, this value was not recorded in the main part of BP_XProfile_Field.
Instead, the requisite option_order checking only happened when a parent field
was being saved via the UI.

Fixes #5472

git-svn-id: https://buddypress.svn.wordpress.org/trunk@8535 cdf35c40-ae34-48e0-9cc9-0c9da1808c22