gimpprocedure: Use less strict typechecking

When executing GimpProcedure.run(), we perform some type checking (and sometimes even do more validation) to make sure that plug-ins are behaving correctly.

The problem with the current implementation was that a plug-in that accepted for example a single GimpDrawable as argument, then trying to call this with a value of GIMP_TYPE_CHANNEL didn't pass the validation, even though the latter is just a subclass of the former.

For that reason, do a proper typecheck using g_type_is_a().