Directly link a MeetingItem to one of the next existing meetings

[gbastien]

It will be interesting for the proposer and the meeting manager to know wich meeting a meeting item is supposed to be presented in.

Add a required referencefield preferredMeeting attribute to the MeetingItem?. A "whatever" option must be available for the proposer, this will give a way to propose a MeetingItem? to any meeting available.