New Feature - Library: Filter by Steam categories

Hi, I added the ability to filter games by categories in Library view.
Todo: 
- The HTML/CSS isn't perfect. The select2 box increases in height when many options are selected, which breaks the layout, so I restricted it to a maximum of 3 at the moment. A workaround, but not really nice.
- Localization: the select2 box offers support already (with lots of translations), have to figure out how to integrate it  - and if it's even worth it...
- Make it a flag in options?