Help: Provide native WebView backend on Mac

Run Qt Creator with environment variable QTC_HELPVIEWER_BACKEND to
* 'native' to get the WebView based one on Mac
* 'textbrowser' to get the QTextBrowser based one

Defaults to use QWebView if QtWebKit is available, or QTextBrowser if
not, like before.

Change-Id: If0660782b18ff3d89301fa7bcaf4e2e2fb69627d
Reviewed-by: Eike Ziller <eike.ziller@digia.com>

src/plugins/help/help.pro

@@ -68,5 +68,14 @@ FORMS += docsettingspage.ui \
     generalsettingspage.ui \
     remotehelpfilter.ui
 
+macx:minQtVersion(5, 2, 0) {
+    DEFINES += QTC_MAC_NATIVE_HELPVIEWER
+    QT += macextras
+    HEADERS += macwebkithelpviewer.h
+    OBJECTIVE_SOURCES += macwebkithelpviewer.mm
+
+    LIBS += -framework WebKit -framework AppKit
+}
+
 RESOURCES += help.qrc
 include(../../shared/help/help.pri)