lang.core: virtual packages
incoming references policy: structure
referenceable concepts/nodes checker
forbid incoming references in sub-concepts intention
forbid incoming references for statements
fix reference in doubleDispatch generator
cannot reference StatementList; children are accessible in the same root only
expressions are not referenceable
fix patterns in test_1@generator
IOperations are non-referenceable
concept functions are non-referenceable
no references to: Creator, xml entities, comments
no references to: Types, StyleClassItem, etc.; fix ReferenceableConceptsChecker
Merge 'origin/master'

Conflicts:
	languages/languageDesign/structure/source_gen/jetbrains/mps/lang/structure/editor/ConceptDeclaration_Editor.java
regenerate after messy merge
fix dummy assert error, introduced by j.m.s.SModel extends openapi.SModel
no references to: Types, CellLayout, warning if INamedConcept cannot be referenced
referenceable concepts
Merge 'origin/master'