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

[mathk]

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:

  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

[HeinrichApfelmus]

Thanks for the pull request!

1. I prefer to not implement VariadicJSParam, but to use an alternative design instead. Namely, every function object in Java has a member apply which can be used to pass multiple arguments as an array, along the lines of

   ffi "aVariadicFunction.apply(null,%1)" arguments

   where arguments :: [Int] is a list of values. For this to work, we need to make lists [a] an instance of the ToJS class.

2. Looks fine to me in principle, but I am not entirely sure which coordinates to use. There were some important issues with pageX not being absolute or relative, but I'll have to look up exactly what the semantics were. I think that mousemove implements it incorrectly, even.