Add proper Modplug icon to Preferences window

This patch also adds the icon in the style of http://openmpt.org to the Mixxx icon template. Also fixes some file permissions in res/images/preferences
![modplug - preferences](https://f.cloud.github.com/assets/4525897/659193/7fc835d0-d66f-11e2-877a-16929558a5e5.png)

PS. Compile with `scons modplug=1` to enable libmbodplug based module tracker support and to make the menu point available in the preferences.