Manual signal handler

[diamondburned]

Right now, some Connect methods generated have a parameter of a slice type,
which is correctly mapped from the GIR declaration, but the goMarshal
converter in pkg/core/glib doesn't actually handle that properly.

There should be an autogenerated API for converting signal handlers as well.
We'll probably need to generate a C callback for each signal, which might
increase compile time by a LOT, but it might also run faster.

It's probably not worth considering adding slice support into goMarshal, given
the amount of hoops that the code converter had to jump through to account for
all the edge cases.