REVIEW: 121717
CHANGELOG: New column "Relative Start Time" which shows how much time has elapsed since the process was started.

Implementation details:
- startTime: The time the process started after system boot. Since Linux
2.6, the value is expressed in clock ticks. See man proc.
- new column also got tooltip and What's this entry
- column is hidden by default (see ksysguardprocesslist.cpp)
- add TimeUtil class
- method secondsToHumanElapsedString is unit tested in chronotest