Handle screensaver activity with it's own class (Bug #12603)

Move the screensaver inhibit, heartbeat, and lock code to its own
set of files we can easily share with xfce4-session. Use the
screensaver's dbus API if it supports it for inhibit and lock
calls. Otherwise, use the heartbeat-command and LockCommand
from xfpm and xfsm if available. Finally, for the lock function,
fallback to trying xdg-screensaver, xflock4, and xscreensaver-command
in that order.