Start transition from QWebKit to QWebEngine

This removes all references to WebKit if cmake option USE_WEBKIT is enabled.
For the user manual it changes it to WebEngine (seems to work for me).

Similar for the Facebook connection (minus a reference to a cookie jar).
This I could not test at the moment, as I wrote this on a train.

Printing does not work, it is a null operation at the moment. Currently,
large parts of of the printing code are commented out as there is no direct
way to access page elements in WebEngine. It seems this needs to be done
via Javascript (with a callback invoked). There is new functionality in
WebEngine to render a view to a PDF file but this needs more work (and
probably some thoughts towards page breaks).

Signed-off-by: Robert C. Helling <helling@atdotde.de>

desktop-widgets/plugins/facebook/facebookconnectwidget.h

@@ -2,7 +2,11 @@
 #define FACEBOOKCONNECTWIDGET_H
 
 #include <QDialog>
+#ifdef USE_WEBENGINE
+class QWebEngineView;
+#else
 class QWebView;
+#endif
 namespace Ui {
   class FacebookConnectWidget;
   class SocialnetworksDialog;
@@ -41,7 +45,11 @@ public:
 	void facebookDisconnect();
 private:
 	Ui::FacebookConnectWidget *ui;
+#ifdef USE_WEBENGINE
+	QWebEngineView *facebookWebView;
+#else
 	QWebView *facebookWebView;
+#endif
 };
 
 class SocialNetworkDialog : public QDialog {