Add support for ELK layout engine

[StephanPirnbaum]

For better layout of PlantUML diagrams, I mostly use ELK (Eclipse Layout Kernel). As ELK is not part of the official PlantUML distribution, using asciidoctor-diagram cannot render such diagrams. This issue can easily circumvented when e.g. using the asciidocotr Maven plugin by simply adding the following ELK libraries as a dependency to the plugin.

<dependency>
    <groupId>org.eclipse.elk</groupId>
    <artifactId>org.eclipse.elk.core</artifactId>
    <version>0.7.1</version>
</dependency>
<dependency>
    <groupId>org.eclipse.elk</groupId>
    <artifactId>org.eclipse.elk.alg.layered</artifactId>
    <version>0.7.1</version>
</dependency>
<dependency>
    <groupId>org.eclipse.elk</groupId>
    <artifactId>org.eclipse.elk.alg.mrtree</artifactId>
    <version>0.7.1</version>
</dependency>
<dependency>
    <groupId>org.eclipse.elk</groupId>
    <artifactId>org.eclipse.elk.graph</artifactId>
    <version>0.7.1</version>
</dependency>

However, when e.g. using the https://github.com/asciidoctor/asciidoctor-intellij-plugin this way of providing additional libraries is not possible. Following the ELK discussions and timeline around PlantUML, I don't see that it will be integrated soon into PlantUML itself.

Therefore, it would be great if asciidoctor-diagram could bring the dependencies to ELK itself and thus support a third layouting option besides GraphViz and Smetana for many tools built on top of asciidoctor-diagram.

[pepijnve]

@ahus1 I think this is feature request for you.

[ahus1]

I had a look during the holidays, and this is what I found:

• For the IntelliJ plugin, you can put extra JARs for rendering the preview in a project specific folder .asciidoctor/lib. See the docs for details. This would then also work for the ELK classes - but that is not sufficient for the current version.
• The current version of ELK applies serviceloader mechanisms to register the layouts, and those use the root class loader for this. I submitted a fix for that, see Use ELK's classloader, not root classloader to discover services eclipse-elk/elk#1109
• As of today, ELK is not included in the very latest version of the PlantUML JARs, still: It is the EPL version, not the LGPL version as this project uses. Not sure how @pepijnve will want to handle this.

In the meantime:

• use https://github.com/ahus1/elk-patched-for-plantuml to build a JAR
• place the JAR .asciidoctor/lib
• confirm the warning to load the JAR (necessary every time you restart your IDE)
• possibly reopen the current AsciiDoc file
• Enjoy - see screenshot below.

As a follow-up, please create an issue in the https://github.com/asciidoctor/asciidoctor-intellij-plugin. Still I don't think I'll include it until the service loader mechanism is working also in custom class loaders. It might be possible to call the registration manually, still I'd like to avoid that. Let's see what the ELK project decides.

[pepijnve]

@ahus1 not sure what I can do about the license situation. If I understand it correctly, we would need to choose between either the LGPL version that contains Ditaa support or the EPL version which contains ELK support.