Implement notification in admin backend and made some refactorings

Block/MaintainedContentWarning.php

Model/BlockInstaller.php

Model/Config/Converter/BlockNodesParser.php

Model/Config/Converter/ContentElementKeyBuilder.php

Model/Config/Converter/PageNodesParser.php

Model/Console/BlockListCommand.php

Model/Console/PageListCommand.php

Model/PageInstaller.php

Model/Query/GetBlockEntryByBlock.php

Model/Query/GetBlockEntryList.php

Model/Query/GetBlocksByBlockEntry.php

Model/Query/GetFirstBlockByBlockEntry.php

Model/Query/GetFirstPageByPageEntry.php

Model/Query/GetPageEntryByPage.php

Model/Query/GetPageEntryList.php

Model/Query/GetPagesByPageEntry.php

Model/Query/HasStoreMatches.php

Model/Query/IsBlockMaintained.php

Model/Query/IsMaintainedContentRequested.php

Model/Query/IsPageMaintained.php