Adding Variadic Js function parameter and add coordinate on drag and drop event

Here is two change in this pull request.

- Variadic js function, To enable variadic js function a new datatype must be used in the ffi call. 
 Typical example: 
 ```haskell
 runFunction $ ffi "aVariadicFunction(%1)" (VariadicJSParam [1, 2, 3])
```

- Adding coordinate when drag and drop event are fire. This is not the ideal solution but I think it is still valuable to have the coordinate attached to the event argument.If you have a better idea on how to implement this feature I would be happy to implement it.

Thanks