Fix infinite display of "Loading..." for some columns.

This especially happened for the second column on the top level
of the font model, which returned an empty result set for the
item data. This previously confused our code as we though that
we are still loading in such cases.

Now, instead add the loading message upon request to the data
structure and blindly return what we have in there. When we get
new data, overwrite what we have.

This works reliably and actually looks simpler as well.
Workaround QTBUG-33321 by converting QIcon to QPixmap.

This is more failsafe anyways, as we cannot be sure to have all the icons
that are available on the remote server side. A pixmap can be rendered
by the client no matter what.

This way, the resource model is properly rendered on the client side.