Split out doBlockLevels() into its own class

Split out doBlockLevels() into its own class

It's independent of the rest of the Parser, but quite intrusive, with
its own instance variables and several private functions. It's also
pretty big (500 lines).

I removed a few functions from Parser here which were always marked
@private in the doc comment, but were inappropriately marked
"public" in the function declaration after migration to PHP 5. I grepped
core and deployed extensions and found no callers.

The helper functions are now all private, and the constructor is
private, with just a single public static entry point, reflecting
the self-contained nature of the module and its lack of hooks.

Change-Id: I1693ed48a9194719611b4afd9d989d44f0610f8d