Increase idle timeout to eliminate log spam

xfconf#28 (closed) created a 5-minute idle timeout for xfconfd but since the Xfce panel saves its settings every 10 minutes xfconfd is being continually reactivated, resulting in dbus adding two lines to the syslog/journal each time. This "noise" makes it difficult to see the important stuff and unnecessarily increases the size of the syslog/journal. It would be great if the delay could be increased to 15+ minutes.