New port: multimedia/QtAV

A cross-platform multimedia framework based on Qt and FFmpeg.
High performance. User & developer friendly. Supports Android,
iOS, Windows store and desktops.

WWW: https://github.com/wang-bin/QtAV

Reviewed by: rakuco, jhale
Approved by: rakuco (mentor)
Differential Revision: https://reviews.freebsd.org/D10186