Prevent adding a function in a section if this function already exists.
Added possibility to use an external template for previewing documents in <rbs-document-list/>: use attribute "preview=true" or define an empty "<preview></preview>" tag. The template is: "Vendor/Plugin/Document/preview-list.twig", and it can be cached with Angular's $templateCache service (in your "admin.js" for example).
Improvements in <rbs-document-list/> for preview:
 - Documents loaded for preview are cached, but reloaded if the user holds the Shift key down.
 - Preview row is visually linked to its origin row.
 Added tip on lists that have preview available. Just to see how the developers will behave :)
Merge pull request #266 from fredericbonjour/Admin

Improvements in <rbs-document-list/> and fixed section-page-function association.