Add a tab to the MeetingItem to manage appendices

[gbastien]

Add an intelligent form that will propose to upload an external file and link it (by a reference) to the meetingitem and that will propose to search for existing plone objects (atbrowserwidget) that the user wish to link to this meetingitem.

The uploaded files are created in the same folder as the point.

These references will be saved in the relatedItems attribute.

[gdelannay]

Management of MeetingFiles? has been done (add, remove). We must still propose a fieldset for selecting existing plone objects (atbrowserwidget)

[gdelannay]

I close this ticket and will reopen another one for implementing the ability to associate annexes that are not simply uploaded files (but it will be for another release).