Drag & Drop + Multiple Map Export

[thetazzbot]

I wanted to contribute a usability fix for your cool utility.

I find my self in rapid iterations between Tiled, Tiled2Unity and Unit. So I looked into the code to determine why my map was not being exported properly. I would just leave Tiled2Unity open and hit the export button when I needed to. Well to my dismay this did not work.

I fixed the code to support a click and go methodology as well as drag and drop multiple tmx files.