Threading visitor implementation for JVisitor.  Now both visit and
endVisit calls follow Node hierarchies to ease generic cases
processing.

Patch by: mike.aizatsky
Review by: spoon


git-svn-id: https://google-web-toolkit.googlecode.com/svn/trunk@6744 8db76d5a-ed1c-0410-87a9-c151d255dfc7