If a type name is unknown, don't create a new StructureType for it, but return mixed

If a QML component inherits from NotFound, typeFromClassName will create a
StructureType whose DeclarationId is "NotFound". The problem arises if NotFound
is later properly declared. declareComponent will therefore create another
StructureType, whose Declaration is "module::NotFound". The two StructureType
having nearly the same name but one having a declaration and not the other will
fight to death and cause random crashes.

This patch also modifies declareComponentSubclass so that special components
(components, modules, signals, ...) see their parent modules and can therefore
use their declarations. This allows "void foo(QtQuick::QModelIndex)" to be
properly recognized.