+/*
+ * call-seq:
+ * edje.on_signal(signal [, source]) { |signal, source| block } => nil
+ *
+ * Registers a callback that will get called when <i>signal</i>
+ * is emitted by <i>source</i>.
+ * If source is nil, "*" will be used instead.
+ * The block is passed two strings, signal and source, which identify
+ * the emission.
+ */
+static VALUE c_on_signal (int argc, VALUE *argv, VALUE self)