index-all page for Groovy JDK DocGenerator

I have added enhanced DocGenerator:

The stylesheet.css is now referenced correctly from all generated html files, before it was either referenced as "stylesheet.css", or as "../../stylesheet.css" which
is wrong for the java.util.regex and org.w3c.dom, and primitive type classes.

A title, and a tool bar is now displayed on the class detail pages, the index page, and the overview page

An index page is created show the methods indexed by the first letter of the method name.

In the attachment is the zip with the changed files, and a patch for DocGenerator.groovy.
I have no patches for the template files, and the stylesheets.css they are all in the zip, too.

For building the stylesheet.css needs to be copied over into the output directory, too.

My motivation was that the Netbeans IDE is now able to use the Groovy JDK api doc like the normal JDK api doc, like searching for methods etc.