AtomBuiltin and decentral Atom box handling

[ghost]

A cleaner version of the Atom implementation used in Image[], that makes it easy to add new Atoms, such as Image[], Graph[], etc. without having to change central code in MakeBoxes.

Two changes:

(1) new AtomBuiltin (from #378); needed as a mechanism to register a builtin-like symbol that is actually an Atom (declaring an Atom directly does not register a name that could be used for constructing it in an Expression, so there always has to be an accompanying Builtin).

(2) moved out logic from class MakeBoxes(Builtin) into a new Atom.atom_to_boxes method, so that new Atoms need not add their logic (and import their symbols) to MakeBoxes but must instead provide an implementation of atom_to_boxes.

In the current implementation, MakeBoxes returns None on unknown Atom types. This PR changes this to raising an NotImplementedError for Atoms not implementing atom_to_boxes. As implementations are provided for Symbol, Integer, Rational, Real, Complex, String, this might only happen if new Atoms are added that do not properly implement atom_to_boxes.

[sn6uv]

Great!