sw: add API that provides fast access to marks of a text node

sw::mark::MarkManager already provides two methods to provide not so slow mark
access:

- some marks (bookmarks, annotation marks) have their own container, so it's
  possible to iterate over only those types, not all of them
- the containers are sorted by the start position of the marks, so it's easy to
  ignore marks that are after the current text node

However, it's not possible to ignore marks ending before the current text node.
This is a problem, as e.g. during ODF export, we have to iterate over every
bookmark for each paragraph, so the operation is not linear.

On the other hand, the start and end position of bookmarks are stored using
SwPosition, and the SwIndex of the SwPosition is already registered in the
SwIndexReg of the SwTxtNode, so the text node sort of already knows what
bookmarks start/end at the current paragraph, it just doesn't known which
position belongs to what mark (if it belongs to a mark).

Fix the problem by adding a pointer to SwIndex that can optionally point back
to the mark that owns it. Also, in the sw::mark::MarkBase methods (which are
the only ones allowed to touch those positions) always set that pointer. With
this, it's possible to get the bookmarks of a node (list of bookmarks which
start or end in the current node) in a much faster way for text nodes.

Change-Id: I7ceeff4dce852b4d72f2a73ae6a2423c7a781e41
sw: use new bookmark API in lcl_FillBookmarkArray() for text nodes

For a document of 1000 pages and having 13000 bookmarks, this helps a
lot on ODF export: 2m51.565s -> 2m23.412s on my machine.

Change-Id: I8c8a7cd2b83a94ab7f2de17e0b4cb449fc4c0e1a