Support for invalid foreign keys

Hi,

This is a new (as in 9.1) feature we have to support in pgAdmin.

From the git log on https://github.com/gleu/pgadmin3/tree/ticket304:

 Add support for invalid foreign key constraints

 This patch adds a checkbox in the dlgForeignKey dialog, so that the
 user can create an "not validated" foreign key. Each foreign key of
 this kind is displayed with the usual icon with a red cross on it. A
 contextual menu allows the user to force the validation of the
 constraint. The user can also do this via the dlgForeignKey dialog.

Patch attached. Comments welcomed :)


--
Guillaume
 http://www.postgresql.fr
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