This adds a way for plugins to tell the cppmodel that for certain files,
the actual contents come from it, instead of reading from the disk.
(For texteditors the cppmodel does that automatically.)

Reviewed-By: Roberto Raggi <roberto.raggi@nokia.com>

replace ExtensionSystem::PluginManager::instance()->getObject<Core::ICore>() by Core::ICore::instance()

This change removes a number of wrong usages of the CppModelManager::documents()/document().