Realtime screen edges gestures for scripted effects

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