StyleSheet now implements IValidIdentifier

The StyleSheet concept now implements IValidIdentifier instead of INamedConcept. This adds constrains
to prevent broken generated code when the name of the stylesheet is invalid for a class.
Merge remote-tracking branch 'coolya/refactor/stylesheet-bug' into 3.2/kDummann

Conflicts:
	languages/languageDesign/editor/source_gen/jetbrains/mps/lang/editor/structure/StructureAspectDescriptor.java