Compare Pane - treatment of duration & problem when adding new metrics

... treat both Time & Duration equally (don't use QTime->toString), but
convert into seconds (since duration can be above 23:59:59 - especially
in Trends view
... table widget looses columns when dynamically adding columns in
Options->Metrics (fixed by setting columnCount to (0) first before
defining the new number of columns)
Merge pull request #953 from Joern-R/pull1

Compare Pane - treatment of duration & problem when adding new metrics