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.
Merge pull request #137 from FlorianFieber/tray_icon_shadow

Use a semitransparent black shadow for embiggened tray icons