Update www/falkon to the latest release.

Release notes https://www.falkon.org/2018/05/08/falkon-301-released/

There's one bit of trickery here, which handles QtWebEngine < 5.10 mixed
with newer Qt -- just the situation the ports tree is in today.

Thanks to Ken Moore for providing partial patches for OPTION KWALLET.

PR: 235681 235680 234692
Reported by: Nuno Teixeira, Ken Moore