alpha: Allow setting the function in ClutterScript
alpha: Allow setting the function in ClutterScript

When defining an Alpha in ClutterScript we should allow setting
the alpha function by using a custom property. This makes it
possible to have both:

  {
    "id" : "behaviour-1",
    "type" : "ClutterBehaviourDepth",
    "alpha" : { "timeline" : "timeline-1", "function" : "alpha_func" },
    ...
  }

And:

  {
    "id" : "alpha-1",
    "type" : "ClutterAlpha",
    "timeline" : "timeline-1",
    "function" : "alpha_func"
  },
  {
    "id" : "behaviour-1",
    "type" : "ClutterBehaviourDepth",
    "alpha" : "alpha-1",
    ...
  }

The latter allows defining a single alpha function for multiple
behaviours.

dlespiau/clutter,djdeath/clutter-multithreaded,rkudiyarov/clutter_osx,Distrotech/clutter,djdeath/clutter,jigpu/clutter,kerrickstaley/clutter-vala,GNOME/clutter,Distrotech/clutter,djdeath/clutter,djdeath/clutter-multithreaded,djdeath/clutter-android,GNOME/clutter,heysion/clutter-clone,kerrickstaley/clutter-vala,jigpu/clutter,kerrickstaley/clutter-vala,jigpu/clutter,Distrotech/clutter,djdeath/clutter-android,djdeath/clutter-android,jigpu/clutter,collects/clutter,jigpu/clutter,dlespiau/clutter,GNOME/clutter,djdeath/clutter,GNOME/clutter,nobled/clutter,djdeath/clutter-multithreaded,rkudiyarov/clutter_osx,ebassi/clutter,collects/clutter,djdeath/clutter,djdeath/clutter,spatulasnout/clutter,ebassi/clutter,nobled/clutter,heysion/clutter-clone,spatulasnout/clutter,djdeath/clutter-android,GNOME/clutter,heysion/clutter-clone,heysion/clutter-clone,dlespiau/clutter,collects/clutter,ebassi/clutter,Distrotech/clutter,rkudiyarov/clutter_osx,kerrickstaley/clutter-vala,djdeath/clutter-android,spatulasnout/clutter,Distrotech/clutter,jigpu/clutter,heysion/clutter-clone,djdeath/clutter-multithreaded,ebassi/clutter,spatulasnout/clutter,collects/clutter,Distrotech/clutter,ebassi/clutter,djdeath/clutter,collects/clutter,nobled/clutter,spatulasnout/clutter,rkudiyarov/clutter_osx