Introduce a common superclass for picker cells

[zoul]

The UISegmentedControl cell and UIPickerView have a lot in common, since they represent the same general idea – being able to pick from a number of options. Would it be possible to extract the common parts to a shared superclass? Things to think about:

• Single/multiple selection, determines the type of the selection path
• Is empty selection possible? If yes, the selection path leads to ItemType?, otherwise ItemType.
• Should the caller be able to customize the item views? How?
• Should the caller be able to customize the selected item view? How?
• Could we parametrize the “style” of the selection control to easily switch between a segmented control, picker view and possibly others?