Pass more context to list_allowed_values()

This is a first stab (setting to needs review to get feedback, but probably needs testing as-well), at adding more context to list_allowed_values(). Use case:

1) Create two content types with a single field
2) Set the field to get the allowed values by function callback.
3) The allowed values should _change_ according to the content type. With the patch, the function callback gets enough context to act accordingly.