Notify extra generators about languages

Some generators can use the any enabled languages to add extra support
in the relevant build tool. One such is Eclipse since there are many
plugins available for various languages.
eclipse: Add natures for Eclipse based on enabled languages

Also adds support for the Java nature if Java is being used.
eclipse: Support custom natures via a global property

This is useful for enabling natures not recognized by the Eclipse
generator directly in a project.
Merge topic 'dev/better-eclipse-language-support' into next

a990722 eclipse: Support custom natures via a global property
51726cc eclipse: Add natures for Eclipse based on enabled languages
4a352d4 Notify extra generators about languages