Add support for an ambient light sensor. (#444)

Connects to net.hadess.SensorProxy, if available, to check for
support, and monitor sensor changes.

When the environment becomes brighter, the backlight increases -
if the environment darkens, the backlight decreases.

- At daemon start. takes current backlight level and ambient level
  and uses it as the base state for future adjustments.
- Any deliberate brightness changes by the user, or by dim-timers
  causes renormalization.
- Upon sensor change:
  - A 3 second stabilization period begins. Any further sensor
    change during this period will restart its timer.
  - After stabilization, the new backlight % is determined. If
    4% of current, nothing is done.
  - If the backlight is to be updated, it is performed gradually
    over a 3 second period.
  - Sensor changes are ignored during transition, and 500ms after.

I tried a couple of iterations of gnome-settings-daemon's
implementation and found them much too sensitive (and distracting).
I've tried to minimize random light 'noise' interfering as much -
temporarily blocking a light source as I move, etc... I don't think
anything can be perfect, and probably not all light sensors will be
made the same, either.

Author: mtwebster

data/org.cinnamon.settings-daemon.plugins.power.gschema.xml.in.in

@@ -1,5 +1,10 @@
 <schemalist>
   <schema gettext-domain="@GETTEXT_PACKAGE@" id="org.cinnamon.settings-daemon.plugins.power" path="/org/cinnamon/settings-daemon/plugins/power/">
+    <key name="ambient-enabled" type="b">
+      <default>true</default>
+      <summary>Enable the ambient light sensor</summary>
+      <description>If the ambient light sensor functionality is enabled.</description>
+    </key>
     <key name="idle-brightness" type="i">
       <default>30</default>
       <summary>The brightness of the screen when idle</summary>