Added basic libxss vala binding, and implement new method (ulong)get_idle_time() in the displays.
vapis/xcrnsaver.vapi: bind the full xcrnsaver header.
xcrnsaver.vapi: bind states and masks.
XlibDisplay: listen to X events and catch XScreenSaver.NotifyEvents.

This permits to fire the new DisplayInterface.idle_changed() signal so that interested listeners
can know where the device is in idle.
DBusService: listen to Display()'s idle_changed signal and set session's IdleHint accordingly.

Now logind is aware when the system is in idle, and thus we can offload the idle action handling to it.
This is a good thing as the user can now select which action to take when the system is idle.
This, of course, "conflicts" with the xscreensaver timer, that needs to be disabled.
Make the idle timeout configurable.

vera now handles the idle timeout, which is configurable from dconf.
Additionally, a new boolean setting "lock-on-idle" handles the lock of the session regardless
of the status of the other ones.
Disable the previously implemented idle-timeout and lock-on-idle settings.

These are still configurable, but vera will not react to them.
The reason for this is pretty simple: this can create conflicts with xscreensaver.
Note: vera will continue to listen to XScreenSaver.NotifyEvent events, and will continue
to set the session's IdleHint accordingly.
0.39
Merge branch 'master' into debian
debian/control: build-depend on libxss-dev.
0.39-1