fix(menu): item click now just Subject, no ReplaySubject

[Tibing]

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Short description of what this resolves:

BREAKING CHANGE:

The NbMenuService not reply the last click event.

To Update:
if you use the knowledge that the last click event is replied you can wrap onItemClick stream in the custom stream based on ReplaySubject.

Closes #695

[nnixaa]

@Tibing do we need to test it in any way? Are you sure it is not breaking anything?