First go at migrating GtkTreeView(Column) to GtkCellArea

First go at migrating GtkTreeView(Column) to GtkCellArea

This is still very much a work in progress, but it renders and more or
less works.  I will be fixing up the details in the very near future.

Important: this commit breaks ABI as it modifies the GtkTreeViewColumn
structure in gtktreeviewcolumn.h.  This is a sealed structure that needs
to be moved to an internal header file, most likely gtktreeprivate.h.