Add item metadata integration hooks to exhibit layouts

[zerocrates]

Imported from Trac ticket #13 reported by jboggs on 2009-07-21:

Add the ability to choose what item metadata gets displayed in an exhibit layout, in addition to normaly showing the item itself (title, description, files, et cetera). This will involve not only modifying the code in each exhibit layout, but also require a way of choosing the metadata to display in the exhibit builder and saving the relationships between that data in the database.