Add icon property and iconChanged signal to QWebEnginePage

The new API makes possible to access downloaded icons via
QWebEnginePage. Thus the QNAM usage for downloading favicons and the
corresponding workaround due to authentication are removed from the
demobrowser.

Change-Id: I9fdcc7ee7673f7caa239d932f20a51c74b24763f
Task-number: QTBUG-51179
Reviewed-by: Leena Miettinen <riitta-leena.miettinen@theqtcompany.com>
Reviewed-by: Kai Koehne <kai.koehne@theqtcompany.com>