Allow adding an item to an existing bound collection

[rjyounes]

Huda will check whether there's a way to do this in the DisplayConfig; otherwise needs a template. (If adding template, then add description to each field as well.)

[rjyounes]

Note that the current workaround is to create the BoundCollection, then go to the BoundCollection and add Items there. However, these must be new Items, since we don't have the lookup in place for existing items (same problem in reverse).