Editable select and nav group 2.0

if one wants to have more than one menue on a page
the "nav group: 'editable_select_name:position'" can be used.

i introduced an editable select for this that is able to
provide this type of control-flow information, so Authors
or Designers can switch between fixed values through the UI.

Example:

Place this editable select within the template:

    {% block menue %}{% editable_select menueposition, options: "top=TopMenue,bottom=Bottom Menue" %}top{% endeditable_select %}{% endblock %}

Now an Author gets an Select Dropdown with 'Top Menue' selected while
editing a page.

Use the following nav tags for top and bottom menues:

    {% nav 'site', class: 'nav top', group: 'menueposition:top' ...  %}
    {% nav 'site', class: 'nav bottom', group: 'menueposition:bottom' ...  %}