title: allowed cTypes in backenlayouts / gridelements not working anymore in TYPO3 7.6?
body: <p>TYPO3 7.6.4
  I have several backendlayouts and also one gridelement for 2col.
  Now I limit the available cTypes for each column like this:
  (just an example)</p>
  
  <pre><code>mod {
  web_layout {
      BackendLayouts {
          Home {
              title = Home
              config {
                  backend_layout {
                      colCount = 1
                      rowCount = 3
                      rows {
                          1 {
                              columns {
                                  1 {
                                      name = Slider
                                      colPos = 1
                                      colspan = 1
                                      allowed = custom_slider
                                  }
                              }
                          }
                          2 {
                              columns {
                                  1 {
                                      name = Content
                                      colPos = 0
                                      colspan = 1
                                      allowed = header, html, shortcut
                                  }
                              }
                          }
                      }
                  }
              }
              icon = icon/path
          }
  </code></pre>
  
  <p>...</p>
  
  <p>For the columns slider it works just fine.
  FOr column content the NewContentWizard shows this 3 Elements, but when I add one, the only element that is allowed (listed in the cType dropdown) is "header" - in that case - so in general its the first cType from allowed.</p>
  
  <p>Does anyone can reproduce or now what's the problem? Otherwise it could be a bug and I'll report it. </p>
  
  <p>Thanks for any feedback or solution!</p>
  
  <p>Kind regards
  Tobi</p>