Document structure for HTML files

[reckart]

Is your feature request related to a problem? Please describe.
We currently have no document structure tree for HTML files. This is somewhat related also to a bug where annotations that appear e.g. in a table or enumeration are not added to a preceding section panel which may be attached e.g. to an h element because the h is not a parent but rather a sibling or uncle.

Describe the solution you'd like
It might be a good idea to introduce an additional set of divs into HTML documents which would properly manifest section boundaries, e.g. start before an h and end just before the next h. We could use those structure elements to build the document structure tree as well as for attaching the section panels.

Describe alternatives you've considered
Work without explicitly manifesting the section boundaries as div elements.