Stub out ApplicationWindow::appendProject.

This method uses the old parsing logic and is due to be replaced. I'm
stubbing it out to allow myself to change the types of any other methods
it calls in turn.

Refs #9970
Move <Matrix> parsing into interfaced stub.

This is the first half of the work required to move <Matrix> parsing
into an IProjectSerialisable interface. It does everything up to the
point of actually parsing the contents of <Matrix>. That ought to come
in the next commit.

Refs #9970