Fixed : Aristo showcase is broken because CPDatePicker

Previously the showcase of Aristo and Aristo2 didn't work because several bugs in the class CPDatePicker.

This PR fixes these problems :
- The first problem was when displaying a textual datePicker, the class tried to calculate different things for the calendar (good optimization as well).
- The second problem was about the calendar and the hands. It wasn't possible to display a PatternColor made with an image in a layer, now we use a PatternImage and the method CGContextDrawImage to draw the hands.