Use item_id for unique delta value in id name

This patch tweaks the way tabs and titles are added to their respective arrays to utilize the field_collection->item_id property instead of the delta / weight of the tab. This may seem trivial but since tab titles and tab ids are generated w/ this value it's allowing for a few things:

• Ability to know which entity a tab / title refers to; I use this data in the rubric module to load contextual links for the tab as an example.
• item_id is unique so that there's better styling support when using nested field collections