[vreg] move base registry implementation to logilab.common. Closes #1916014 A new logilab.common.registry module has been created with content from * cw.vreg (the whole things that was in there) * cw.appobject (base selectors and all). In the process, we've done some renaming: * former selector functions are now known as "predicate", though you still use predicates to build an object'selector * hence `objectify_selector` decorator is now `objectify_predicate` * the top level registry is now `RegistryStore` (was `VRegistry`) Also there is no more need for the @lltrace decorator. On the CubicWeb side, the `selectors` module has been renamed to `predicates`. There should be full backward compat with proper deprecation warnings.

  <table class="attributeForm" style="width:100%;"
	 tal:attributes="id tab_id | nothing;
			 class tab_class | nothing;">
    <tr tal:iter="widget lines">
      <th class="labelCol" tal:content="structure python:widget.render_label(entity)">attrname</th>
      <td tal:define="error python:widget.render_error(entity)" style="width:100%;"
          tal:attributes="class python:error and 'error' or nothing">
	<div tal:replace="structure error">error message if any</div>
	<div tal:replace="structure python:widget.edit_render(entity)" >widget (input, textarea, etc.)</div>
	<div tal:replace="structure python:widget.render_help(entity)">format help if any</div>
      </td>
    </tr>
  </table>