title: Nested categories/InlinePanel(s) in wagtail
body: <p>I struggle to implement something like "nested categories": </p>
  
  <pre><code>PageA:
    - Cat1
      - SubCat1
      - SubCat2
      - ...
    - Cat2
      - SubCat1
    - ...
  </code></pre>
  
  <p>All categories and subcategories should be orderable and editable by an editor. </p>
  
  <p>My guess was something like this: </p>
  
  <pre><code>class CategoryTestPage(Page):
      content_panels = Page.content_panels + [
          InlinePanel('categories')
      ]
  
  
  class Category(Orderable,ClusterableModel,models.Model):
      page = ParentalKey(CategoryTestPage, related_name='category')
      category = models.CharField(max_length=250)
  
      def __str__(self):
          return "%d %s" % (self.id, self.category)
  
      panels = [
              FieldPanel('category'),
              InlinePanel('subcategory')
      ]
  
  class SubCategory(Orderable,models.Model):
      category = ParentalKey(ProjektOrdnung, related_name='subcategory')
      subcategory = models.CharField(max_length=250)
  
      def __str__(self):
          return "%d %s" % (self.id, self.subcategory)
  
      panels = [
              FieldPanel('subcategory')
      ]
  </code></pre>
  
  <p>But this results in <code>'CategoryForm' object has no attribute 'formsets'</code>. It seems nested <code>InlinePanel</code>s are the problem?</p>
  
  <p>Further I need this "hierarchical taxonomy" for assigning some of these categories/subcategories to other pages: </p>
  
  <pre><code>PageB:
      - has Cat1
        - has SubCa2
      - ...
  </code></pre>
  
  <p>... which looks a lot like hierarchical tags...</p>
  
  <p>Any ideas how to implement this or what's wrong with my implementation?</p>
  
  <p>Kind regards, 
  tombreit</p>
  
  <p>PS: I'm on wagtail 1.2rc1</p>