Add Charm::weekDifference and pointlessly rewrite exisiting work

The monthly timesheet was crashing for me because end of December is week #1, and the beginning is week #48, so I made a function to calculate week differences.  
  
Then I realized that 5 weeks was hardcoded into the monthly timesheet, so I made it dynamically calculate there too. Then I realized I hadn't pulled from KDAB/Charm for a very long time and that the 5 week hardcoding was already fixed.  
  
Then I said my day is done, so here you go. This is only really useful for the Charm::weekDifference.