When SlickEdit is the foreground app, go to the "X11" menu, and select Preferences. (Command + comma). On the pref dialog, under the Input category, uncheck the "Enable keyboard shortcuts" option.
You'll now notice that Command + Q now correctly closes just SlickEdit, instead of shutting down all of X11, and Command + W will close the active code window, not the entire application. And your cut/copy/paste operations will now work.