Asset browser to view and convert sprites

This addresses #2532 and #2865 as well as showcases how powerful our current widgets already are.

For our pixel artists who are alienated by command line interfaces. This streamlines the graphical modding process with a nice quick preview that shows everything put into the mounted mods folders without any sequence files to be set up.

The workflow
* Watch how the existing sprite look like
* Export one to PNG to fiddle around with it
* Import an edited one by converting it to an SHP

is now completely GUIfied with some error handling.

![uL97h5D](https://f.cloud.github.com/assets/756669/437959/7c9b6f34-b0b4-11e2-995a-9d603422b02a.png)

It will also help me with #2566 in the future.