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/api_generic.rst

@@ -16,6 +16,9 @@ classes:
 * :class:`SPIDevice` represents devices that communicate over an SPI interface
   (implemented as four GPIO pins)
 
+* :class:`InternalDevice` represents devices that are entirely internal to
+  the Pi (usually operating system related services)
+
 * :class:`CompositeDevice` represents devices composed of multiple other
   devices like HATs
 
@@ -31,6 +34,9 @@ There are also several `mixin classes`_:
 * :class:`SharedMixin` which causes classes to track their construction and
   return existing instances when equivalent constructor arguments are passed
 
+* :class:`EventsMixin` which adds activated/deactivated events to devices
+  along with the machinery to trigger those events
+
 .. _mixin classes: https://en.wikipedia.org/wiki/Mixin
 
 The current class hierarchies are displayed below. For brevity, the mixin
@@ -47,6 +53,10 @@ Next, the classes below :class:`SPIDevice`:
 
 .. image:: images/spi_device_hierarchy.*
 
+Next, the classes below :class:`InternalDevice`:
+
+.. image:: images/other_device_hierarchy.*
+
 Next, the classes below :class:`CompositeDevice`:
 
 .. image:: images/composite_device_hierarchy.*
@@ -60,15 +70,18 @@ Base Classes
 ============
 
 .. autoclass:: Device
-    :members: close, closed
+    :members: close, closed, value, is_active
 
 .. autoclass:: GPIODevice(pin)
     :members:
 
-.. autoclass:: CompositeDevice
+.. autoclass:: SPIDevice
     :members:
 
-.. autoclass:: SPIDevice
+.. autoclass:: InternalDevice
+    :members:
+
+.. autoclass:: CompositeDevice
     :members:
 
 Input Devices
@@ -77,9 +90,6 @@ Input Devices
 .. autoclass:: InputDevice(pin, pull_up=False)
     :members:
 
-.. autoclass:: WaitableInputDevice
-    :members:
-
 .. autoclass:: DigitalInputDevice(pin, pull_up=False, bounce_time=None)
     :members:
 
@@ -128,3 +138,6 @@ Mixin Classes
 .. autoclass:: SharedMixin(...)
     :members: _shared_key
 
+.. autoclass:: EventsMixin(...)
+    :members:
+