Introduce BP_XProfile_Field_Type::display_filter(), and implement in BP_XProfile_Field_Type_Datebox.

The new display_filter() method makes it easy for field types to customize the
appearance of its values. In the case of Datebox fields, this means formatting
the MySQL- or UNIX-formatted date in the format of bp_format_time().

Fixes #5630

git-svn-id: https://buddypress.svn.wordpress.org/trunk@8547 cdf35c40-ae34-48e0-9cc9-0c9da1808c22