Combine candidate icons for a page into a single icon

For the Widget API the QIcon returned by QWebEnginePage::icon() function
contains all the candidate icons for the current page.

For the Quick API the QQuickWebEngineFaviconProvider provides the best
quality icon for the requested size from the candidates.

Task-number: QTBUG-51179
Change-Id: I42b8427f957e2f2fc745dd0111bedcc71b577216
Reviewed-by: Allan Sandfeld Jensen <allan.jensen@theqtcompany.com>