Added:
* Pile: new class derived from Plane, which implements all the
`ncpile_*` calls
* Plane: `get_parent` (`ncplane_parent`)
* Plane: protected constructor for use by `Pile` (or other derived
classes which cannot provide a valid `ncplane*` when invoking parent
constructor.
* Plane: `set_plane` for use by the derived classes above.
Changed:
* Plane: `to_ncplane` is a `const` method now.
