Support managing referenced libraries

[Vigilans]

Fix #174, microsoft/vscode-java-pack#94

Based on:

• Refactor UpdateClassPathJob to provide a more general interface  eclipse-jdtls/eclipse.jdt.ls#1267
• Refactor onDidChangeConfiguration handler in favor of listener pattern #212
• Support context value of view nodes #215

VS Code will take care of the file watching and searching work by providng a setting:

"java.dependency.referencedLibraries": string[] | {
  "include": string[],
  "exclude": string[],
  "sources": { [binary: string]: string }
}

...with the following extended behaviors before sending to jdt.ls:

• If specified setting is an array, it will be expanded into a full object, with the original content as include field.
• Jars in lib/ but not included will be appended to exclude field. It is to ensure jars not listened by VS Code don't get shown in referenced libraries — only one source of truth. (This is not an elegent workaround, waiting for better alternative...)
• A match of foo.jar and foo-src.jar (not necessarily in the same folder) will be added to sources mapping.

[testforstephen]

what's the status for this PR?

[Vigilans]

Some important modifications are saved in my Linux PC. May I rebase to the master branch and update the PR after going back home tonight?

[jdneo]

Simply push might not work in some scenario, like:

if the original setting is:

"java.project.referencedLibraries": {
        "include": [
            "lib/**/*.jar"
        ],
        "exclude": [
            "lib/junit-platform-console-standalone-1.5.2.jar"
        ]
    }

where the junit-platform-console-standalone-1.5.2.jar is the only jar in the lib folder.

Now if I re-add the lib folder into the referenced libs, the exclude part won't be updated.