Drag-and-drop support for VST, SF2

Adds drag-and drop support for SF2 and DLL file extensions.
![image](https://cloud.githubusercontent.com/assets/6345473/3046484/6e609d4e-e129-11e3-8680-c317e2c6f4d2.png)

http://www.youtube.com/watch?v=eBQg2vVnjPQ

The new pixmaps are placeholders until Vesa is able to add official artwork.
`soundfont_file.png`, `vst_plugin_file.png`

Per #730