Fixed: Allow Interface Builder to set CPComboBox enabled state

Previously, the enabled state of a CPComboBox could not be set in Interface Builder.

This commit allows Interface Builder to set the enabled state of a CPComboBox.

A modified test in Manual/CPComboBoxTest is included. This test simply adds a new disabled combo box to an existing window, and then checks to see if it is disabled, printing a descriptive message to the Javascript console.
Merge pull request #1868 from ahankinson/fix-cpcombobox-ib-disable

Fixed: Allow Interface Builder to set CPComboBox enabled state
Fixed: nib2cib of combo box parsed enabled state twice.

It's sufficient to read the _enabled state in the superclass NSCell, no need to read it again in the subclass.

Refs #1868.