check any reference in delete confirmation dialog

[thet]

Support for any zc.relation refercences being checked by delete_confirmation_info dialog, not only references linked in text.

/cc @jensens @mauritsvanrees

[thet]

Updating of modified relations happens elsewhere. E.g.:
https://github.com/plone/plone.app.relationfield/blob/master/plone/app/relationfield/event.py
and
https://github.com/zopefoundation/z3c.relationfield/blob/master/src/z3c/relationfield/event.py

[jensens]

LGTM

[pbauer]

@thet this change needs to be reflected in the docs. So far the README says : "Whenever an object that is referred to by another one via an or tag is going to be deleted, a confirmation form is presented to the user." This in no longer true. Also it should also mention what happens (I think z3c.relationfield.event.breakRelations is called).

[thet]

@pbauer : #53