feat: Display non-java resources in the Project explorer

[jdneo]

Fix #145

The new explorer will looks like this:

[jdneo]

Performance impact

Test case

After project petclinic imported, Open the file PetClinicIntegrationTests.java to let Explorer fetch the nodes and sync the position.

Without this PR:

request	total time
java.resolvePath	566
java.getPackageData	69

With this PR:

request	total time
java.resolvePath	354
java.getPackageData	85

The time cost of java.resolvePath, it's usually ranging from 300~600ms

[CsCherrYY]

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:

[jdneo]

Here is a case about non-Java project, given Gradle sample project

Filed an issue to track: #736

[CsCherrYY]

LGTM overall. One small thing: can we adjust the order of source sets? Showing main/test source sets before the generated source sets would be better.

after:

before: