Add autocompletion for CMS_HEADER contents

By now there is autocompletion for

  • variables in output channels
  • Rules channel (to a certain extent)
  • Form channel

I request full autocompletion for the contents of the CMS_HEADER area. Looking up the options of the navigation CMS_FUNCTION is annoying and any enterprise CMS with a built-in editor should offer autocompletion to its developers. Thinking forward, this is a requirement for distributed template development in external IDEs.