Add the external binding feature

This commit introduce two new features:

  <urn:AGL:widget:provided-binding>
    <param "name"="public-name" "value"="relative-path-to-the-binding"/>

  <urn:AGL:widget:required-binding>
    <param "name"="public-name" "value"="extern">
    <param "name"="relative-path-to-the-binding" "value"="local">

TODO: manage security by setting correct Smack label to the exported files

Bugs-AGL: SPEC-1439
Bugs-AGL: SPEC-1663

Change-Id: I70aad7d523ece7a2ae0058b79708a02ef81144e2
Signed-off-by: José Bollo <jose.bollo@iot.bzh>