Accidentally exported functions

[Jonathan Blandford <jrb redhat com>]

There are four functions:

gtk_tree_view_column_cell_render
gtk_tree_view_column_cell_draw_focus
gtk_tree_view_column_cell_focus
gtk_tree_view_column_cell_set_dirty

that should be made private.  I meant to move them ages ago, but forgot
to )-:.  LXR indicates that no one is using them, and no one should.
The first two will have possibly indeterminate behavior, and the last
two will screw up internal state.  Despite the API freeze, I'd like to
make them private and remove them to avoid someone thinking they have a
different behavior and calling them.

Thanks,
-Jonathan