Rename new brightness fading GConf settings for clarity

Avoid short forms, add unit suffixes and brief explanations.
Rename old display GConf settings

Remove superfluous "path" information from macro names and comments.
Actual key names are not changed for the sake of backwards compatibility.
Rename mdy_brightness_set_fade_target_* functions for clarity

Avoid short forms.
Add unit suffixes to fade duration variables

Also break long lines and remove excess whitespace.
Do not use fade-to-black during active calls

Needed to make stray touch input events from ear/chin less likely
when placing the phone at ear.

Also clarify why fade-to-black length should be kept short.
Clarify use and implementation of priority boosting during fading
Fix typo
Use rounding instead of truncating when calculating brightness fade value
Use helper function for common fade timer cleanup tasks