Sheets: Fix date/time/datetime save, load, parse and formatting
Time: Added a new Time class that can handle negative- and positive duration in addition to clock time.
Localization: Added formats and parsing of duration.
DateTime: Using similar solution as date and time
Number dialog: Making the code more robust by storing format type with the combobox for date, time and datetime.
Load/save: Handle duration, neagtive and positive.. Handle fractions of seconds to better precision than milliseconds.
Unit tests: Unit test for Time class added. All tests now passes, but many of the tests that depended on KLocale functionality that QLocale does not have, have been marked XFAIL so they are not forgotten.