Possibility to implement realtime screenedges gestures in scripted effects,
implement it in the windowsaperture show desktop effect.
* Expose registerRealtimeScreenEdge to JavaScript, the callback will be
a JS function.
* Add the concept of freezeInTime() in the animation js bindings,
it will either create an animation frozen at a given time or freeze a running animation
that can be restored and ran to completition at any time
* add an edges property only for showdesktop as it's not directly on the effect configuration