Introducing createAssociationsDrawerXMLElement

This method allows us to create the proper XMLElements to add a link
to the drawer without having a Field instance.

This also allows extension developers to leverage the current, default
implementation of the markup while controlling the value to be displayed.

This changes also makes prepareAssociationsDrawerXMLElement call
createAssociationsDrawerXMLElement.
Fixing part of #2130

This change makes sure the publish page won't crash if no visible field
is found. It display the entry id (system:id) instead.
Merge pull request #2137 from DeuxHuitHuit/issue-2130

Fix for association drawer with no visible fields, RE: #2130