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/overwrites columns when dynamically adding columns in Options->Metrics (fixed by setting columnCount to (0) first before defining the new number of columns - if not, first columns are fine, but the added one get lost, or overwrite e.g. the last column)