Auto-completion of Maven configuration properties inside beans

Some plugins define configuration inside classes, for instance the `<resources>` element here https://github.com/asciidoctor/asciidoctor-maven-plugin/blob/master/src/main/java/org/asciidoctor/maven/AsciidoctorMojo.java#L148.

In eclipse the properties are shown making the configuration easier, but not in IntelliJ.

Ideally, properties should be based on setter methods to allow plugin designers to keep properties independent for internal/real variables.

Is it possible to implement this?

I have checked the plugin.xml descriptor and I've identified 2 possible cases to support:

- Lists/Collections: this is tricky, because the descriptor only says `<type>java.util.List</type>`. I imagine eclipse uses reflection on the plugin jar to identify the class from the mojo and then get the properties.

- Classes: this is simple, in this clase the <type> contains the cannonical name of the class. This should be easier, but still requires some reflection usage.