Help: Move viewer backend management from plugin to local help manager

which poses as a settings class.

Change-Id: Icb01f0d4750c2d197875954a648ae7358c4c217a
Reviewed-by: David Schulz <david.schulz@qt.io>

src/plugins/help/localhelpmanager.h

@@ -26,6 +26,7 @@
 #pragma once
 
 #include <coreplugin/helpmanager.h>
+#include <utils/optional.h>
 
 #include <QMetaType>
 #include <QMutex>
@@ -33,6 +34,8 @@
 #include <QUrl>
 #include <QStandardItemModel>
 
+#include <functional>
+
 QT_FORWARD_DECLARE_CLASS(QHelpEngine)
 
 class BookmarkManager;
@@ -40,6 +43,15 @@ class BookmarkManager;
 namespace Help {
 namespace Internal {
 
+class HelpViewer;
+
+struct HelpViewerFactory
+{
+    QByteArray id;
+    QString displayName;
+    std::function<HelpViewer *()> create;
+};
+
 class LocalHelpManager : public QObject
 {
     Q_OBJECT
@@ -90,6 +102,10 @@ public:
     static int lastSelectedTab();
     static void setLastSelectedTab(int index);
 
+    static QByteArray defaultViewerBackend();
+    static QVector<HelpViewerFactory> viewerBackends();
+    static HelpViewerFactory viewerBackend();
+
     static void setupGuiHelpEngine();
     static void setEngineNeedsUpdate();