Help: Add litehtml based viewer backend

For CMake add litehtml installation path to CMAKE_PREFIX_PATH
For qmake pass litehtml installation path via LITEHTML_INSTALL_DIR qmake
variable

Release build of litehtml is recommended.

The litehtml backend is used by default when available, you can force
QTextBrowser again with the environment variable
"QTC_HELPVIEWER_BACKEND=textbrowser".

Some things are not implemented yet:
- Text search
- Context menu
- Shift-drag to extend existing selection

Change-Id: I79f989e5fe2063de2e9832abbed19b24d7a1a1fe
Reviewed-by: hjk <hjk@qt.io>

src/plugins/help/helpplugin.cpp

@@ -46,6 +46,9 @@
 #include "textbrowserhelpviewer.h"
 #include "topicchooser.h"
 
+#ifdef QTC_LITEHTML_HELPVIEWER
+#include "litehtmlhelpviewer.h"
+#endif
 #ifdef QTC_MAC_NATIVE_HELPVIEWER
 #include "macwebkithelpviewer.h"
 #endif
@@ -455,10 +458,15 @@ HelpViewer *HelpPlugin::createHelpViewer(qreal zoom)
     using ViewerFactory = std::function<HelpViewer *()>;
     using ViewerFactoryItem = QPair<QByteArray, ViewerFactory>; // id -> factory
     QVector<ViewerFactoryItem> factories;
-#ifdef QTC_WEBENGINE_HELPVIEWER
-    factories.append(qMakePair(QByteArray("qtwebengine"), []() { return new WebEngineHelpViewer(); }));
+#ifdef QTC_LITEHTML_HELPVIEWER
+    factories.append(qMakePair(QByteArray("litehtml"), [] { return new LiteHtmlHelpViewer(); }));
 #endif
-    factories.append(qMakePair(QByteArray("textbrowser"), []() { return new TextBrowserHelpViewer(); }));
+#ifdef QTC_WEBENGINE_HELPVIEWER
+    factories.append(
+        qMakePair(QByteArray("qtwebengine"), []() { return new WebEngineHelpViewer(); }));
+#endif
+    factories.append(
+        qMakePair(QByteArray("textbrowser"), []() { return new TextBrowserHelpViewer(); }));
 
 #ifdef QTC_MAC_NATIVE_HELPVIEWER
     // default setting