Disable editing of picker row fields. fixes #102

Modifies PickerRow and DateRow to:

  * hide the caret while the text field is being edited
  * not show the clear button
  * not allow any text changes coming from a keyboard

Thoughts: looking at this patch, it's probably clear that

  * DateRow should inherit from PickerRow
  * PickerRow should not inherit from StringRow, and should not have a text field, but rather be a direct subclass of Formotion::RowType::Base and have a UILabel.