Refs #4059 Converting dozens of plugins/* classes to use Namespaces, \Piwik\Plugins\*

core/API/DataTableManipulator.php

core/API/DocumentationGenerator.php

core/API/Proxy.php

core/API/Request.php

core/Access.php

core/AssetManager.php

core/Common.php

core/Controller.php

core/DataTable/Filter/ColumnCallbackAddColumnPercentage.php

core/DataTable/Renderer.php

core/Db/Schema.php

core/FrontController.php

core/Loader.php

core/Menu/MenuAbstract.php

core/Period.php

core/Piwik.php

core/Plugin.php

core/PluginsArchiver.php

core/PluginsManager.php

core/ReportRenderer.php