Show non Java Projects in the Java Projects explorer

[jdneo]

          Here is a case about non-Java project, given Gradle sample project (https://docs.gradle.org/current/samples/sample_building_java_applications.html), the current Java projects view shows only project `app` since it's a java project, and ignore the root project `demo` since it's a non-java project. 

Since we have supported displaying non-java resources, we can also consider showing the non-java project in the project explorer. Given eclipse package explorer:

Originally posted by @CsCherrYY in #730 (comment)