Fix html5 video

Deploy the built ffmpegsumo library and make chromium find it
by overriding the content::DIR_MEDIA_LIBS path with the deploy
location.
We deploy the plugin to $$[QT_INSTALL_PLUGINS]/qtwebengine.

Change-Id: I0aeb37fccd2af62b23f8e6405041655e1f66b7aa
Reviewed-by: Zeno Albisser <zeno.albisser@digia.com>
Reviewed-by: Pierre Rossi <pierre.rossi@gmail.com>