Broadly speaking, this change adds the concept of <vfunc> to the .gir.
The typelib already had most of the infrastructure for virtual functions,
though there is one API addition.

The scanner assumes that any class callback slot that doesn't match
a signal name is a virtual.  In the .gir, we write out *both* the <method>
wrapper and a <vfunc>.  If we can determine an association between
them (based on the names matching, or a new Virtual: annotation),
then we notate that in the .gir.

The typelib gains an association from the vfunc to the function, if
it exists.  This will be useful for bindings since they already know
how to consume FunctionInfo.