www/qutebrowser: update to 1.2.1

• Update to 1.2.1;
• Switch MASTER_SITE to CHEESESHOP and install documentation this distfile provides;
• Add WebEngine backend support and drop pkg-message that tells only WebKit is supported;
• Add missing audio/alsa-plugins dependency for sound support;
• Drop deprecated optsuffix;
• Install .desktop file.

Reviewed by: jrm
Differential Revision: https://reviews.freebsd.org/D14723