Adds filtering toggle to "Recent" to filter recently added, played, created, listened to...

This sorting helps me better keep track of what I'm currently listening to.

Up to you if you want to upstream it.  It's a bit outside of solving the particular problem of color schemes, but this extension is really a pretty good platform for any UI enhancements to the Google Play Music web app.

![Screenshot 1](https://cloud.githubusercontent.com/assets/786691/5408420/00289fe6-818f-11e4-8b9e-46ae8603cf93.png)