Ubuntu: improved loading of error dialogs

I eventually figured out that if you passed the dialog as a URL you can use the
caller parameter (which in our case is the appWindow) which drives the dialogs
placement.

The third parameter can hold a series of properties configuring the popup to be
opened. This can be any property defined by the popup.

REVIEW: 117357
Ubuntu: messagelistpage display message reversely

We're now going through the ThreadingMsgListModel which can reverse the sorting
order for us.

The let's-make-ThreadingMsgListModel-available-to-QML should be probably moved
to a proper QML plugin at some point, perhaps with other C++ classes -- that is
yet to happen.

BUG: 332985
REVIEW: 117346