Add a function to QDate to calculate the difference to another date

Qt currently lacks date calculation functions (like e. g., to mention a nice example, Python has), esp. the possibility to calculate the difference to another date other than the days between.

In my case, I needed the years, months and days between two dates. It's always quite tricky to do such calculations (but they are often needed I think), so it would be nice if some function to do this calculation was added to QDate ditrectly (like QDate::differenceTo(const QDate &other) or such).

The function I implemented is quite short, but it took me quite some time to get it right (and I hope it calulates correctly). Maybe you want to use this (probably as an inspiration).