Add support for fixed time brightness fading

Below here, please rename the constants and GConf keys to make them easier to understand:

```
MCE_GCONF_BRIGHTNESS_FADE_DEFAULT_DURATION_MS (brightness_fade_default_ms)
MCE_GCONF_BRIGHTNESS_FADE_DIMMING_DURATION_MS (brightness_fade_dimming_ms)
MCE_GCONF_BRIGHTNESS_FADE_ALS_DURATION_MS (brightness_fade_als_ms)
MCE_GCONF_BRIGHTNESS_FADE_BLANK_DURATION_MS (brightness_fade_blank_ms)
MCE_GCONF_BRIGHTNESS_FADE_UNBLANK_DURATION_MS (brightness_fade_unblank_ms)
```
(feel free to remove `_DURATION`, though,  if that makes the constant names too long in your opinion, but add `_MS` / `_ms` as the suffix, and spell out `default` and `dimming` (`def` could also mean definition, `dim` could also mean dimension)

And for the comments, I think it's clear that it's the "Path to the [...] fade duration", maybe add some info in the comment when it's used (e.g. when is the default used, when dimming, when ALS, etc..).