Fixed: support for dates (CPDatePicker) in CPPredicateEditor

Before this commit, row templates and views belonging to them were copied via archiving/unarchiving.
This was causing problems when the view was not completely CPCoding compliant, like for CPDatePicker.

With this commit, template views use CPCopying (-copy method) when available.
Also added CPCopying for CPDatePicker as a category in CPPredicateEditor.

Test: in CPPredicateEditorTest, on the right panel, choose a keypath, operator, and select 'Dates' in the popup for the right expression.

Fixes #2041