Add location (x,y) to the filesDropped event

[notlion]

Uses [sender draggingLocation] and converts to top-left content-relative coordinates. I had to kill trailing spaces in here as well but I made it an extra commit in case you want to cherry-pick. But then.. no one likes those freeloading spaces, do they?

[deanm]

I committed basically this done a tiny bit differently.

https://github.com/deanm/plask/commit/224b293cb8bbaf34b944e17d9da49bdc9c790693

There is also an example now 010_image_drop.js

Thanks!