This is the first in a series of refactors that will make object.cpp
easier to maintain and eventually save memory by using two different
private classes for instances and prototypes.

This commit changes the ObjectInstance struct to be a C++ class. (Once we
do that it becomes quite obvious that the class should be split into an
instance and a prototype version; it has two constructors, and many
methods that apply only to one or the other.) The way we distinguish
instances and prototypes is that the GObject *m_gobj member is null in
the prototype; but again this indicates why we need the refactor, since
occasionally m_gobj is also null during object disposal.

We try to keep the diff minimal by writing the methods in the same place
where the equivalent C code was already written. This unfortunately
leaves the code somewhat disordered compared to the class body
declaration.

The code is now a bit more readable due to small accessor methods such as
is_prototype(), is_custom_js_class(), etc. These methods will likely be
inlined by the compiler.