Add a "List of Changes" in the Navigator. The list is updated only when the document...

Add a "List of Changes" in the Navigator. The list is updated only when the document structure is reset or when the update button is clicked. Some work is needed to track more finely individual changes as we do for sections.

Changes::addToToc(): New method to insert changes sorted by authors. The symbols 0x2702 and 0x270d are used to represent deletion and insertion.

TocBackend::item(): new method to lookup for a given item.

git-svn-id: svn://svn.lyx.org/lyx/lyx-devel/trunk@26639 a592a061-630c-0410-9148-cb99ea01b6c8