New port: graphics/qt5-wayland

Marketing blurb [1]:

QtWayland is a Qt 5 module that wraps the functionality of Wayland.
QtWayland is separated into a client and server side. The client side
is the wayland platform plugin, and provides a way to run Qt applications
as Wayland clients. The server side is the QtCompositor API, and allows
users to write their own Wayland compositors.

This is mostly needed at the moment to make upstream KDE-CI happy, therefore
we don't wire it into the metaport devel/qt5.

It requires a little change to devel/qt5-qmake, as we needed to modify the
installed bsd.conf to know about wayland/egl.

Created together with Adriaan de Groot <groot@kde.org>.

Reviewed by: rakuco, groot_kde.org
Differential Revision: https://reviews.freebsd.org/D11744