RFR: JDK-8216319: Refactor JavadocTester to allow more on-by-default checkers; add A11YChecker

[Jonathan Gibbons]

Please review a medium-simple change to refactor JavadocTester to allow 
more on-by-default checkers, and to introduce a new checker

The nested classes for HtmlParser and LinkChecker are moved out of 
JavadocTester to become top-level classes; a new interface HtmlChecker 
is introduced between HtmlParser and LinkChecker, and a new subtype of 
that interface is added.

The A11YChecker is currently simple ... when enabled, it checks for 
content outside of a region in HTML 5 files, and it checks for 
out-of-order headings. We may want to add additional checks in future.

Architectural limitations in JavadocTester mean it is simpler/easier to 
read each file for each checker, meaning that if both checkers are 
enabled, the files will be read/parsed twice. This is in contrast to 
DocCheck where each file is read once and the checkers are run in 
parallel. However, relatively speaking the number and size of the files 
is small (compared to, say, the JDK API docs), and the overhead is not 
noticeable, and preferable to any more substantial refactoring of 
JavadocTester at this time.

A11yChecker is not enabled by default at this time: it finds lots of 
errors related to out-of-order headings; however, it is expected that 
when we fix that issue, A11yChecker will then be enabled by default.

-- Jon

JBS: https://bugs.openjdk.java.net/browse/JDK-8216319
Webrev: http://cr.openjdk.java.net/~jjg/8216319/webrev.00/