Nav Doc link checks (remote resources + reading order)

[rdeltour]

disallow remote links in toc/landmarks/page-list nav …

A new message NAV_010 (ERROR) is reported when a toc, landmarks,
or page-list nav contains links to remote resources.

Fix #890

check that toc & page-list nav are in reading order

New Message:

• NAV_011 (ERROR) is reported when the links in toc and page-list
nav (in the Navigation Document) do not follow the reading order
  (i.e. spine order + order of IDs in the Content Documents).

Checking logic for NAV_011:

1. collect the nav links as references in XRefChecker
2. check the reading order consistency in XRefChecker.checkReferences
   (ran after all documents have been parsed and validated)

Internal changes:

• In the PathUtil utilities:
  • rename the removeAnchor method to removeFragment
  • add a new getFragment method
• In the OPFItem class:
  • for spine items, record their position in the spine
  • add a new public getSpinePosition() getter
• In the XRefChecker class:
  • add a new getResource(path) public method to get an OPFItem
    for a given path.
  • add a new getAnchorPosition(id) method to the internal Resource
    class.
  • some registered references are duplicates of already-registered
    references, with a specific reference type (e.g. a Nav Doc link is
    registered both as a HYPERLINK and as a NAV_TOC_LINK): these
    subtypes are now checked in separate private methods, to keep the
    main checkReference(Reference) method pristine.
  • add a new checkReadingOrder(Queue, int, int) private method which
    processes a queue of references (along with the last known spine
    position and fragment position) and check they are in reading order
• add new tests for the Navigation Document reading order check
• fix existing tests

Fix #888