Add loading of artwork in other formats as well as .png

This is a patch I put out on Sourceforge some time ago, but it was forgotten somehow. It adds functionality to load images in all formats a QImageReader can handle. 

Also includes pixmap caching from old-master, that part's not my doing.

SVGs for 0.4.x era B&B editor loaded and resized to ridiculously wide:
![svgload](https://cloud.githubusercontent.com/assets/6411463/3376883/de3f1c06-fbd9-11e3-88c3-9a92abb1472d.png)