Easily enable proprietary codecs

While we can't ship this by default in our binary packages in the future,
it would be nice for users who can afford it to easily build with support
for h264 among others.

And for our private use, that means we can now watch videos from Vimeo.

This simply requires passing WEBENGINE_CONFIG+=proprietary_codecs to qmake
when building.

Change-Id: Ib2274a7cfd053aaa3d38ac18e360310691e11534
Reviewed-by: Zeno Albisser <zeno.albisser@digia.com>
Reviewed-by: Jocelyn Turcotte <jocelyn.turcotte@digia.com>