With AllTray you can dock any application with no native tray icon (like 
Evolution, Thunderbird, Terminals) into the system tray. A highlight feature 
is that a click on the "close" button will minimize back to system tray.  
It works well with Gnome, KDE, Xfce 4.*, Fluxbox, and WindowMaker.

NOTE: If you have installed one of the GNOME distributions for Slackware,
then you can safely remove the "--disable-gconf \" line in the SlackBuild.