Pass is_popup_var result in item_editor context, so that e.g. richtext init templates can work with earlier feincms/django versions.

This allows external libraries to be compatible to previous versions of feincms by using a construct like:

{% with FEINCMS_POPUP_VAR|default:"pop=1" as popup_var %}
  ..."some_url?{{ popup_var }}"...
{% endwith %}
Merge branch 'next' of github.com:feincms/feincms into next
Properly separate tree and item editor JavaScript and CSS