XProfile Admin: Add placeholder attribute to group name and field name input fields.

This improves the UX when having to add a new profile group or profile
field on the "Users > Profile Fields" admin page.

Fixes #5859.

git-svn-id: https://buddypress.svn.wordpress.org/trunk@8991 cdf35c40-ae34-48e0-9cc9-0c9da1808c22