Easy extra fields

Makes defining and filling extra fields easier and faster, because:

• Helper to define extra fields, in the normal hook_field_extra_fields()
• Automatic callback to fill those extra fields with specific entity type + bundle + field content
• Only load those extra fields if the current display has then activated. This is very useful for expensive/heavy content, like forms and theme calls.
• Helper to define fake fields to fill those extra fields with, to make output markup consistent with real Fields.