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Backlight helper: Improve function comments
Backlight helper: Use shorter function and constant names
Backlight helper: Add support for modifying the brightness switch
Rename ENABLE_BRIGHTNESS_CONTROL to HANDLE_BRIGHTNESS_KEYS
Settings dialog: Rename option for handling brightness keys
Add callback code for handle brightness keys option
Hook up code for handling brightness keys option
Implement brightness switch handling in common/xfpm-brightness

This adds the necessary utility functions to read and write the kernel
brightness switch setting.
Install property to track current brightness switch value

Similar to xfpm-power we set this up in xfpm-backlight.
Install property for restoring original brightness switch value

On most machines, especially those using the video module backlight, the kernel
will already have hooked up the brightness keys. If the user wants to use
xfce4-power-manager to control the keys, then we need to disable this module
setting to avoid conflicts like brightness lowered or raised twice on key
presses.

Of course, it would be nice if the power manager restored the original setting
when it gives control back to the kernel or on exit. To be able to do this,
it needs to remember the value.

It is best to do this in a xfconf setting so it survives crashes. This way,
power manager has a chance of restoring the original setting even after a
crash, so that the user will not have to fix it manually.

There are three possible settings, read from
/sys/module/video/parameters/brightness_switch_enabled:

 1 = kernel brightness switch setting is enabled (value 'Y')
 0 = kernel brightness switch setting is disabled (value 'N')
-1 = kernel brightness switch unavailable or could not be read
Only run code for setting brightness switch if it has been initialized
Setup brightness switch on startup
Enable/disable brightness switch depending on handle keys option at startup
Toggle brightness switch when user changes handle brightness keys option