Add authentication token support for wayland windows

Add authentication token support for wayland windows

For compositors that support it, the wayland clients can associate themselves
with an auth token, specified by WL_AUTHENTICATION_TOKEN env var, or by
directly specifying it in the wayland client plugin.

Change-Id: I74a50a27c7c61c2b2cf1e09868618f36edc94cb1
Reviewed-by: Samuel Rødal
Reviewed-on: http://codereview.qt.nokia.com/116
Reviewed-by: Paul Olav Tvete <paul.tvete@nokia.com>