Added "size" and "composited" options.

The "size" option allows for the icon size to be set
(default 16).

When the "composited" option is enabled (default) we use
compositing to draw the system tray icons. If a compositing
manager is running this will result in the system tray being
partially transparent. This option does nothing if there
is no RGBA visual available or the display does not support
compositing.