Implement Injector->handleEnd, with lots of refactoring for injector.

Previous design of injector streaming involved editability only to start, empty
and text tokens, because they could be safely modified without causing formedness
errors.  By modifying notifyEnd to operate before MakeWellFormed's safeguards
kick into effect, it can be converted into a handle function, allowing for
arbitrary modification of end tags.

This change involved quite a bit of restructuring of the MakeWellFormed code,
including the moving of end of document tags to inside the loop, so rewinding
on those tags would be functional, increased reuse of the end tag codepath by
code that inserts end tags (as they could be changed out from under you), and
processToken modified to have an extra parameter to force re-processing of
a token if the original token was an end token.

We're not exactly sure if handleEnd works at this point, but the important
talking point about this refactoring is that nothing else broke. Also, a number
of convenience functions were moved from AutoParagraph to the Injector
supertype (specifically: forward, forwardToEndToken, backward, and current).

Signed-off-by: Edward Z. Yang <edwardzyang@thewritingpot.com>

NEWS

@@ -12,6 +12,9 @@ NEWS ( CHANGELOG and HISTORY )                                     HTMLPurifier
 3.1.2, unknown release date
 # Using %Core.CollectErrors forces line number/column tracking on, whereas
   previously you could theoretically turn it off.
+# HTMLPurifier_Injector->notifyEnd() is formally deprecated, and has changed
+  behavior slightly (end tokens it sees are not guaranteed to exist). Please
+  use handleEnd() instead.
 ! %Output.AttrSort for when you need your attributes in alphabetical order to
   deal with a bug in FCKEditor. Requested by frank farmer.
 ! Enable HTML comments when %HTML.Trusted is on. Requested by Waldo Jaquith.
@@ -36,6 +39,11 @@ NEWS ( CHANGELOG and HISTORY )                                     HTMLPurifier
 ! Track column numbers when maintain line numbers is on
 ! Proprietary 'background' attribute on table-related elements converted into
   corresponding CSS.  Thanks Fusemail for sponsoring this feature!
+! Add forward(), forwardUntilEndToken(), backward() and current() to Injector
+  supertype.
+! HTMLPurifier_Injector->handleEnd() permits modification to end tokens. The
+  time of operation varies slightly from notifyEnd() as *all* end tokens are
+  processed by the injector before they are subject to the well-formedness rules.
 - Fix two bugs in %URI.MakeAbsolute; one involving empty paths in base URLs,
   the other involving an undefined $is_folder error.
 - Throw error when %Core.Encoding is set to a spurious value. Previously,