Prefer generating class pointers over values

[diamondburned]

This is a follow up from issue #25 and commit d16c0d15:

The record pointer change is expected to be followed up with a similar
change for classes. This will make signal generation slightly more
accurate as well, since both of those types are always passed by
pointers.

[diamondburned]

Example signal:

func (*gdk.DisplayManager).ConnectDisplayOpened(f func(display gdk.Display)) glib.SignalHandle                                                                               

should be

func (*gdk.DisplayManager).ConnectDisplayOpened(f func(display *gdk.Display)) glib.SignalHandle