Compare Pane - sorting improvements

... when sorting first and adding new items, table data is partly empty
and when you next click on item GC dumps (fixed by de-activating the
sorting while refreshing the QTableWidget)
... date fields did not sort due to different formats strings used in
"toString" and "fromString" conversion / added info to translators
... duration and time fields did sometimes not sort (dependent on
time/duration having hours or not)
Merge pull request #952 from Joern-R/pull6

Compare Pane - sorting improvements / fix SEGV