Fixed: a disabled CPMenuItem could still have a selected background

Previously it was possible to have a highlighted CPMenuItem for a disabled CPMenuItem. (To get that, select an item, close the menu, disabled the item manually and then open the menu again).
Now it is not possible to get this behavior.
Merge pull request #1976 from Dogild/HighlightedDisabledMenu

Fixed: a disabled CPMenuItem could still have a selected background
Test: disabling a highlighted menu item should deselect it.

Refs #1762.