I've decided that it is isn't feasible to make cell areas runtime-settable
in the time we have left before 3.0, therefore, I'm going with the
approach to allow init() functions to instantiate the default cell area
and issue a warning if a construct property is ignored.
This is not ideal, but it keeps existing icon view and combo box
subclasses working.
https://bugzilla.gnome.org/show_bug.cgi?id=639139
