For most documents, headings that go 10 deep are more than adequate.
But for some very large documents, and for applications that use OOo Writer
headings in sophisticated ways, 10 levels of headings is not enough.

Currently, the maximum depth of headings is "baked in" in a number of ways:

 * The Navigator only shows entries to that depth 
   (even if deeper headings are defined)

 * Navigator promote/demote operations only work to that depth
   (deeper headings, if defined, remain in place)

 * Tab/Shift-Tab movements in the editor do not apply