Add override_class_handler.

Author: Susurrus

src/signal.rs

@@ -7,6 +7,7 @@
 use libc::c_void;
 
 use gobject_ffi::{self, GCallback};
+use glib_ffi::GType;
 use source::CallbackGuard;
 use translate::ToGlibPtr;
 
@@ -19,6 +20,11 @@ pub unsafe fn connect(receiver: *mut gobject_ffi::GObject, signal_name: &str, tr
     handle
 }
 
+pub unsafe fn override_class_handler(signal_name: &str, instance_type: GType, handler: GCallback) {
+    gobject_ffi::g_signal_override_class_handler(signal_name.to_glib_none().0, instance_type, handler);
+}
+
+
 unsafe extern "C" fn destroy_closure(ptr: *mut c_void, _: *mut gobject_ffi::GClosure) {
     let _guard = CallbackGuard::new();
     // destroy