RFR: JDK-8274311: Make build.tools.jigsaw.GenGraphs more configurable

[Iris Clark]

On Fri, 24 Sep 2021 23:07:33 GMT, Mandy Chung <mchung at openjdk.org> wrote:

> GenGraphs tool generates the module graph. It currently supports the configuration via javadoc-graphs.properties. However, `make/jdk/src/classes/build/tools/jigsaw/javadoc-graphs.properties` only documents two properties. It should be updated to cover all configurable properties.
> 
> There are a couple other properties not configurable such as nodesep and node margin. This extends the configuration to allow to set additional properties. 
> 
> This also fixes `requiresMandatedColor` in javadoc-graphs.properties to light gray to match the default configuration in the implementation, i.e. the color of the edge to java.base.  It seems a bug that was unnoticed until Alex and Iris spotted it.

Marked as reviewed by iris (Reviewer).

-------------

PR: https://git.openjdk.java.net/jdk/pull/5690