Don't discard highlighted annotations when merging paragraphs

All text nodes that are children of grouping elements should be counted
as ODF content.

Fixes #666.
Merge pull request #667 from peitschie/odf-empty-fixup

Don't discard highlighted annotations when merging paragraphs
Re-highlight annotations whenever an edit occurs.

* To make it less expensive, a method has been added
to AnnotationViewManager to refresh not all annotations
but only the highlights.

* This is used as a scheduled redraw task inside SessionView
to not make it a CPU hog.

* The destroy mechanism for SessionView has been restructured
to accomodate multiple asynchronous destructors.
Fix closure annotations for DomUtils.getNodesInRange and
OdfUtils.getTextNodes.
Never completely re-wrap annotation highlights, only wrap
those text nodes which should be within but are as of yet
unhighlighted.