Use the proper source model for the SystemTray unread count.
Review Request #121403 - Created Dec. 9, 2014 and submitted
This used to iterate over mails as well (i.e. all contents of the ETM), which is pretty useless _and_ slow. I detected this with heaptrack, as it triggered hundred thousands of temporary allocations of invalid Collections and those in turn allocated CachePolicies. Turned out, the model did not check whether it can convert the source data to a Collection, but just blindly converted it. Thus, a default constructed Collection was returned for non-collection items. Which is slow. Now, we just iterate over the collection model, which will always return valid Collections for us - much better!