Theme engine: make it possible to merge properties in contributed views

contributed widgets views can be used by different modules, for instance:

<view name="nuxeo5 tree explorer" template-engine="jsf-facelets">
<format-type>widget</format-type>
<template>incl/tree_explorer.xhtml</template>
</view>

may require "faceted-search.css" contributed by a separate add-on:

<view name="nuxeo5 tree explorer" template-engine="jsf-facelets">
<resource>faceted-search.css</resource>
</view>

it should be possible to add resources (css, js) to an existing view without modifying the original definition, by merging the <resource> properties.