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.