SF2/VST Drag-and-Drop Support

This minor patch closes #730 by allowing the dragging-and-dropping of VSTs as well as SoundFont files into the Song-Editor and Beat/Bassline Editor.

Please view the functionality in this video:  http://youtu.be/TM-s7NX5vK4

**Note:**  Since my previous pull request from the parent branch isn't accepted yet, this branch erroneously shows those 95 changes.  Please scroll past them.
![screen shot 2014-05-17 at 12 55 18 am](https://cloud.githubusercontent.com/assets/6345473/3004154/9e8cd6de-dd7f-11e3-88e9-800dbe1129f7.png)