Ambient light control - alter brightness for all monitors

[digitaltrails]

Add an ambient-light-control slider for manual lux input. Adjusting the slider would apply the per-display lux-brightness profiles, which means the control would simultaneously adjust brightness of all the displays.

The slider would include indicators of typical lux ranges:

A first alpha cut of this has been pushed. Refinements to follow.

[digitaltrails]

Problems:

1. Sort out how to turn the option on. I'm currently uses lux-slider-enabled setting, but that seems unnecessary.
2. Refine the look and feel, panel titles, tick marks, icons, etc.
3. Spinbox input isn't working.
4. Smooth out interactions with the existing automatic lux control (can be delayed a bit - why?).

[digitaltrails]

f1d7728:

• The Ambient Light Control option now turns on/off via the lux device selector in the lux-meter dialog.
• Some progress on the look and feel

[digitaltrails]

• implemented persistence of the manual ambient slider value.
• added log scale.
• refactored the LuxDisplay code to react to signals from the LuxMeters - makes for instant display of new values.

[digitaltrails]

Turn off smoothing of lux values when manually read from the slider (otherwise the UI response lags, and considerably if the change is large in magnitude).

Consider also turning off ignoring small changes in resultant brightness - not sure if that's necessary.

[digitaltrails]

Released now, will raise any improvements as new issues.