Use symbolic icons where appropriate

You might not agree with this change so I'd like your opnion.The [Gnome HIG](https://wiki.gnome.org/Design/HIG/IconsAndArtwork) mentions how the majority of icons should use symbolic icons. They are much cleaner, obvious what they mean, and scale better. I only did the toolbar and playlist area, the file browser should probably use it too. Here is an example of what it looks like:

![Example](https://cloud.githubusercontent.com/assets/798838/2686296/555939d8-c1f7-11e3-8f0c-9ed575cde57e.png)