do not save a const reference to the user data in the info dialog

libcockatrice_utility/libcockatrice/utility/days_years_between.h

@@ -0,0 +1,8 @@
+#include <QDateTime>
+
+inline static QPair<int, int> getDaysAndYearsBetween(const QDate &then, const QDate &now)
+{
+    int years = now.addDays(1 - then.dayOfYear()).year() - then.year(); // there is no yearsTo
+    int days = then.addYears(years).daysTo(now);
+    return {days, years};
+}