Fix for bug #142 & xfce4-session#115 re incorrectly using interface 'org.freedesktop.ScreenSaver'

Bug #142 (closed): Fixed problem /w attempting to call non-existent method 'Lock' on D-Bus interface 'org.freedesktop.ScreenSaver' by modifying the logic that selects how to lock the screen, to no longer attempt to use this interface incorrectly, but to invoke 'xflock4' instead.

This is accomplished by moving the case conditional for 'SCREENSAVER_TYPE_FREEDESKTOP' from where the interface would be incorrectly called to just above the case conditional for 'SCREENSAVER_TYPE_OTHER', causing the D-Bus interface 'org.freedesktop.ScreenSaver' to be considered after all the other D-Bus interfaces, and to be processed like 'SCREENSAVER_TYPE_OTHER', thereby invoking 'xflock4'.

(Also helps fix xfce4-session#115 (closed))