Close button sends client to the tray, statically linked Qt for OSX.
See the Changelog for list of features and fixes currently only available in the nightly builds.
Thank you for broaching the subject :) I intend to add a small 'Support Soulseek!' at the top right corner of the client window at some point. This will take you to a web page that allows you to make a donation, and once your privileges are activated, the button will display your remaining days of privileges left instead.