LCD hardware fading

[Richard Hughes]

Some LCD panels (like my Toshiba) can switch instantly from one
brightness state to another, so going from brightness 1 to brightness 10
gives a flash to the screen.
In g-p-m, the solution to this was to set the brightness like :
1,2,3,4,5,6,7,8,9,10 with a 100ms gap in between. This gives a nice
"fade" rather than a sudden jump of values.

Some laptops, namely IBM laptops have hardware in the LCD panel that
does a hardware fade between states, and so the 1,2,3... interim steps
are not required, and should not be done [1] as it makes the display
fade in an undesirible way.

I made just the IBM laptops use hardware dimming in g-p-m, to fix the
problem, but it would appear that also Apple laptops also have dimming
hardware.

To stop an un-maintainable list of laptops growing in g-p-m, and for
easy debugging, I propose a laptop_panel.hardware_fade (bool) key that
would signify if the LCD panel hardware has a hardware dimmer.

Laptops could be added (or the list refined) trivially in the fdi file,
which would mean any application changing the brightness knew what to do
(i.e. hardware or software dimming).

Patch attached, please review.

Richard.

[1] https://bugzilla.redhat.com/bugzilla/show_bug.cgi?id=173382
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