(bug 23393) (bug 45803) Parser: Fix whitespace handling within headings

* HTML <hN> headings containing line breaks are now handled correctly
  (bug 23393).

* Whitespace within == Headline == syntax and within <hN> headings is
  now non-significant and not preserved in the HTML output (bug
  45803).

Change-Id: I0f2d81dd0b2f7742c5cdb6b7d2cc58a15d3f1029
Merge "(bug 23393) (bug 45803) Parser: Fix whitespace handling within headings"