CMS-3055 Make it possible to remove one and only FormItemSet occurrence when minimum occurrences is 0

It is possible to remove las FormItemSet occurrence if minimum is 0. Collapse button is hidden in this case.
CMS-3036 Unify code for local event listening

Additional refactoring, some classes were missed during last refactoring
Merge pull request #602 from enonic/CMS-3055

CMS-3055 Make it possible to remove one and only FormItemSet occurren…