Make it possible to configure a filter on the list of users displayed by the User suggest widget

Users & groups suggest widget should make it possible to set a filter on the list of returned users (use case on groups is less obvious). Various kind of filters are interesting, by order of priority:

• Filter users belonging to a specified list of groups only.
• Filter users having specific values on some of their attributes
• A seam component's variable that would contain a list.
• A static closed list of users

This feature would be very useful for configuring assignment tasks in workflows, enabling users to assign tasks to a controled list of users.