Implemented new abstract pattern for "find*" methods using a "findEntities"

Implemented new abstract pattern for "find*" methods using a "findEntities" method in the base class "ImplContainer".

To achieve this a "children()" method was added to "Sections" and "Sources" that wraps "sections()" or "sources()" methods.
For naming consistency "getMultiple" was renamed to "getEntities".