Big Picture button and Web Scrollbars

Created and changed the big picture button image file to something that works in the same position used by regular steam.
Still yet to figure out why it remains in a controller shape, but hey

Should look something like this, but a lot smaller
![24 - 16-50-52](https://cloud.githubusercontent.com/assets/5154676/4384464/286abf90-43b7-11e4-9a02-65a3758c61eb.png)


Edited the chromium scrollbars to something that fits the rest of the client a bit more
![DeshouSTEAM@16-24-37 | 24-9-14](https://cloud.githubusercontent.com/assets/5154676/4384266/bdb1aefa-43b3-11e4-8537-081e20ccca17.png)