Comment 29 for bug 380844

A few notes about current progress:

Source file
 * Implicit anchors are not saved in the source file, they are generated from their labels when page is parsed.

Indexing
 * Anchor id and label are indexed to provide autocompletion in Insert Link dialog.

Parsetree
 * Elements with anchors (only headings for now) have "anchor" attribute.

TextBuffer
 * Tags with anchors (only headings for now) have "anchor" attribute.
 * Editing of page can cause inconsistencies in anchors, anchors become dirty (TextBuffer.anchors_dirty = True).
 * TextBuffer.cleanup_anchors() finds and repairs dirty anchors.
 * Dirty anchors don't affect editing except autocompletion of local anchors ("#anchor"), therefore anchors have to be cleaned up when Insert Link dialog is opened.
 * Dirty anchors have to be cleaned up before page is saved and reindexed.

Modified anchors
 * Links to local anchors (within the current page) are repaired in TextBuffer.cleanup_anchors()
 * TextBuffer.cleanup_anchors() emits "anchors-modified" signal and stores changes in a dict self.modified_anchors (old_id => new_id)

TODO:
 * Support for image links to anchors
 * Index links to anchors
 * Repair links to anchors outside the current page
 * Detect anchor moved to another page
 * Show anchor type (heading, object, ...) in anchor selector