Fix iconChanged signal and add favIconLoad QML test

If WebContentsDelegateQt::DidUpdateFaviconURL was not called after
a page load, we need to reset the favicon URL. We can use the
navigation entries to store favicon related information.

Change-Id: I7bcfbd46c176fabce319eba32c379a293f8ebba6
Reviewed-by: Peter Varga <pvarga@inf.u-szeged.hu>
Reviewed-by: Pierre Rossi <pierre.rossi@gmail.com>