Style notifications if the window manager supports it.

Old:

![tomahawk-notify-old](https://f.cloud.github.com/assets/70274/160841/fb35a794-7778-11e2-832a-24cb1b1b46fa.png)

New:

![tomahawk-notify-new](https://f.cloud.github.com/assets/70274/160842/0156d1d4-7779-11e2-8287-f4264e8d15ae.png)