Add support for additional chooser fields

[rgriffith]

Currently, it is only possible to map to file choosers. For added flexibility, it would be nice to be able to map to all of the possible chooser types, or those deemed most appropriate (ie do we need a symlink chooser).

Use case: migrated content contains links to SSI's and those includes are created in Cascade as XHTML Blocks. The Data Definition for the pages contains a block chooser for these include blocks.

[bradical]

3+ years old. Will icebox and leave open. Perhaps if we get another dev to help own migration tool they can take on improvements like this.