incorrectly changes the class path order in Order and Export tab #519

Class path order is fixed and can be changed manually on Order and
Export tab. Addition of any element to class path should be added at the
end of the list and previous elements should be at their location.

Fixed redundant notification to adapter in case of removing all elements
in the Libraries List.

Fixes https://github.com/eclipse-jdt/eclipse.jdt.ui/issues/519