Add Lock() D-Bus method and make xflock4 a simple wrapper around it

libxfce4ui!97 (merged) and 7c846f57 brought behaviors of XfceScreensaver and xflock4 close enough to take the step, and thus have only one locking behavior on Xfce, despite several entry points: shell script, D-Bus, instantiation of XfceScreensaver.

This allows to keep xflock4 which is used in various places in the Xfce code base and for a long time by users, without needing to make it a binary instantiating XfceScreensaver.

As for the D-Bus method, it can be used in other contexts, for example where a proxy on org.xfce.SessionManager is already used for other things, again avoiding instantiating XfceScreensaver and thus depending on libxfce4ui.

Fixes: #148 (closed), #149 (closed), #120 (closed) via libxfce4ui!99 (merged)
Related: libxfce4ui!97 (merged)