Review Request: JDK-8173303: Add module-subgraph images to main platform documentation

[Mandy Chung]

Jon and I work on this patch as the first step to enable module
graphs for inclusion in the javadoc.  It includes a @moduleGraph
taglet, an updated GenGraphs tool, and also updates javadoc of 
module-info.java to include @moduleGraph.  I also take the 
opportunity to add simple javadoc to a few JDK modules preparing
for JEP 299 that will generate module summary page for JDK modules.

Webrev at:
  http://cr.openjdk.java.net/~mchung/jdk9/webrevs/8173303/webrev.00/

Here shows some sample of the javadoc with module graph images:
  http://cr.openjdk.java.net/~mchung/jdk9/module-graph-docs/overview-summary.html

Click on a module and it will show a module graph icon. Mouse over
it will show the full size image.  java.base is a special case 
that has no hovered image since it has one node and the full
size image is already shown.

The build work to generate PNG file from .dot file will be 
done separately [1].
 
Mandy 
[1] https://bugs.openjdk.java.net/browse/JDK-8176785