Use a semitransparent black shadow for embiggened tray icons

On X11, Psi previously added an opaque gray "shadow" to tray icons which
looks out of place on dark or colored panels. A semitransparent black
shadow should look cromulent on any panel color.

![Comparison of Tray icon shadows](https://cloud.githubusercontent.com/assets/917394/2875733/2428e4cc-d425-11e3-9018-fe4e9650e55c.png)
On the left is the previous shadow, on the right the new shadow.