Improve Messages component - add different inboxes (other boxes)

[slaFFik]

There is a cool idea $box = 'inbox', but currently there is no ability to add other "boxes", which messages will be associated with.

Would be nice to have such feature.

[DJPaul]

I think it's nice to think of these as "labels", like how gmail does it (so you could categories a message into >1 views), but I think ultimately this depends on what we all decide we want the vision to be for the messaging feature.

Probably we have this buried in Future Release milestone somewhere already, but moving this one in, just in case.

[slaFFik]

I believe #5193 will cover this task as well. So I propose to close this task and work on #5193 at least for now (working on it).

[tw2113]

going to listen to @slaFFik and close this one in favor of #5193