Fixed: CPDatePicker textual and graphical doesn't work in a CPTableView

Previously, when using a textual or graphical CPDatePicker in a CPTableView Cappuccino crashed.
This PR fix this issue by adding the subviews of a CPDatePicker in the method layoutSubviews instead of in the initialization.

This PR add another feature on the graphical CPDatePicker, the button next and previous month are now in the mode continuous as in Cocoa.
Merge pull request #2193 from Dogild/DatePickerTableView

Fixed: CPDatePicker textual and graphical doesn't work in a CPTableView