The source/values toolkit

Me and my big mouth. No sooner do I declare the base classes "relatively
stable" than I go and mess around with it all again. Anyway, this is the
long promised set of utilities to make source/values more interesting.
It includes a few interesting little utility functions, a whole bunch of
examples and introduces the notion of "pseudo" devices with no (obvious)
hardware representation like a time-of-day device.

This necessitated making the event system a little more generic (it's
not exclusive the GPIO devices after all; no reason we can't use it on
composite devices in future) and by this point the mixins have gotten
large enough to justify their own module.

The pseudo-devices are a bit spartan and basic at the moment but I'm
sure there'll be plenty of future ideas...

docs/images/gpio_device_hierarchy.dot

@@ -9,7 +9,6 @@ digraph classes {
     node [color="#9ec6e0", fontcolor="#000000"]
     Device;
     GPIODevice;
-    WaitableInputDevice;
     SmoothedInputDevice;
 
     /* Concrete classes */
@@ -17,9 +16,8 @@ digraph classes {
 
     GPIODevice->Device;
     InputDevice->GPIODevice;
-    WaitableInputDevice->InputDevice;
-    DigitalInputDevice->WaitableInputDevice;
-    SmoothedInputDevice->WaitableInputDevice;
+    DigitalInputDevice->InputDevice;
+    SmoothedInputDevice->InputDevice;
     Button->DigitalInputDevice;
     MotionSensor->SmoothedInputDevice;
     LightSensor->SmoothedInputDevice;