New: Added feature to select a set of dates with a shift click in CPDatePicker

Previously it wasn't possible to select a set of dates with a shift click, now we can.
This works only when the mode CPRangeDateMode is set.
It has the same behavior as in Cocoa.
Merge pull request #2131 from Dogild/datePickerShiftClick

New: Added feature to select a set of dates with a shift click in CPDatePicker