This generates broken code, but the problems are dealt with in the
next few commits.

There are cases where multiple methods are indicated in the gir files
to map to the same native function. If these are generated into the same
class, the generated code breaks. An example is when a non-virtual method
is defined globally, and also in an interface. Both are generated into the
namespace-wide catch-all class. This commit prevents the generation of these
duplicates.

An example of a relocated method is a non-abstract method found in an
interface, which is generated into the namespace-wide catch-all class
instead.

Mostly a mechanism to actually call the pointed virtual functions,
and the parsing of anonymous callbacks. This is a quick commit to
share these changes, and really needs cleaning up before pushing
to master...

Not sure when the default changed, or if it changed at all, but
I recently noticed there were no line numbers in exception stack
traces. This solves the problem.