[Sound applet] Add slider for seeking

Attempts to address #1640

This needs to be tested with a bunch of different media players.

Also, indent or no indent?

![screenshot-cinnamon-2013-02-01-131435](https://f.cloud.github.com/assets/164802/119792/e486a508-6cb4-11e2-9e80-311de87a0701.png)