Make Core independent from QtHelp

We don't want various plugins to depend on the Help plugin,
but we also do not want Core to depend on QtHelp.
For example when turning the Help plugin off, documentation should
actually no longer be registered through QtHelp. So we need
parts of the interface in Core, which must then be delegated
to the actual implementation in Help.

As positive side-effects the interface in Core will be slimmer,
and the code in the Help plugin can later be simplified, too,
because then we don't have the "Core" and the "Gui" help engines
separated in different plugins anymore, which should remove the
need for some setup indirections.

Task-number: QTCREATORBUG-20381
Change-Id: I634c5811c45d6a3dfd6ddc682cae270e38384cbf
Reviewed-by: hjk <hjk@qt.io>

src/plugins/coreplugin/helpmanager_implementation.h

@@ -0,0 +1,49 @@
+/****************************************************************************
+**
+** Copyright (C) 2016 The Qt Company Ltd.
+** Contact: https://www.qt.io/licensing/
+**
+** This file is part of Qt Creator.
+**
+** Commercial License Usage
+** Licensees holding valid commercial Qt licenses may use this file in
+** accordance with the commercial license agreement provided with the
+** Software or, alternatively, in accordance with the terms contained in
+** a written agreement between you and The Qt Company. For licensing terms
+** and conditions see https://www.qt.io/terms-conditions. For further
+** information use the contact form at https://www.qt.io/contact-us.
+**
+** GNU General Public License Usage
+** Alternatively, this file may be used under the terms of the GNU
+** General Public License version 3 as published by the Free Software
+** Foundation with exceptions as appearing in the file LICENSE.GPL3-EXCEPT
+** included in the packaging of this file. Please review the following
+** information to ensure the GNU General Public License requirements will
+** be met: https://www.gnu.org/licenses/gpl-3.0.html.
+**
+****************************************************************************/
+
+#pragma once
+
+#include "helpmanager.h"
+
+namespace Core {
+
+namespace HelpManager {
+
+class CORE_EXPORT Implementation
+{
+protected:
+    Implementation();
+    virtual ~Implementation();
+
+public:
+    virtual void registerDocumentation(const QStringList &fileNames) = 0;
+    virtual void unregisterDocumentation(const QStringList &nameSpaces) = 0;
+    virtual QMap<QString, QUrl> linksForIdentifier(const QString &id) = 0;
+    virtual QByteArray fileData(const QUrl &url) = 0;
+    virtual void handleHelpRequest(const QUrl &url, HelpViewerLocation location = HelpModeAlways) = 0;
+};
+
+} // HelpManager
+} // Core