The marshallers we use for the signals are declared in a private header,
and it stands to reason that they should also be hidden in the shared
object by using the common '_' prefix. We are also using some direct
g_cclosure_marshal_* symbol from GLib, instead of consistently use the
clutter_marshal_* symbol.
ClickAction adds "clickable" semantics to an actor. It provides all
the business logic to emit a high-level "clicked" signal from the
various low-level signals inside ClutterActor.
