ChoiceBoxSelectionModel: Issues with Separator selection.

ChoiceBoxSelectionModel's methods needs to be corrected in handling the Separator and SeparatorMenuItem.
1. selectPrevious and selectNext do not consider SeparatorMenuItem.
2. selectFirst and selectLast needs to be overridden similarly as select selectNext and selectPrevious.
3. In JDK-8088261, the behavior of select() was changed, Now if a Separator is selected then the selection is cleared, which also does not seem correct. Considering the aspect of JDK-8088261 we should find a way to fix select method too.