Add empty stub for an aggregated property model and integrate it.

Doesn't show anything yet apart from the right row count. This will
eventually replace the static, dynamic and non-QObject property views
with a single aggregated view.
Add index mapping and implement data().
Implement flags() and headerData().
Implement setData() and dataChanged().

That's basically it, since we currently don't support adding/deleting
dynamic properties the missing row change signal handling isn't really
a problem yet.
Make sure all property source models have the same column count.