script: Allow connecting signal to ClutterState states

One of the uses of a ClutterState state machine along with ClutterScript
is to provide a quick way to transition from state to state in response
to signal emitted on specific instances.

Connecting a real function, in code, to a specific signal does not
improve the ease of use of ClutterScript to define scenes.

By adding a new signal definition to the current one we can have both a
simple way to define application logic in code and in the UI definition
file.

The new syntax is trivial:

  {
    "name" : <signal name>,
    "state" : <state machine script id>,
    "target-state" : <target state>
  }

The ClutterState instance is identified by its script id, and the target
state is resolved at run-time, so it can be defined both in
ClutterScript or in code. Ideally, we should find a way to associate a
default ClutterState instance to the ClutterScript one that parses the
definition; this way we would be able to remove the "state" member, or
even "style" the behaviour of an object by replacing the ClutterState
instance.

The implementation uses a signal emission hook, to avoid knowing the
signal signature; we check the emitter of the signal against the object
that defined the signal, to avoid erroneous state changes.

clutter/clutter-script-private.h

@@ -88,8 +88,12 @@ typedef struct {
   gchar *name;
   gchar *handler;
   gchar *object;
+  gchar *state;
+  gchar *target;
 
   GConnectFlags flags;
+
+  guint is_handler : 1;
 } SignalInfo;
 
 void property_info_free (gpointer data);