(feat) Service: Implement NodeQuery.

[hastebrot]

This PR will enhance the nested methods API for Node retrieval with a chained methods API. Nested methods will be used for common tasks, chained methods for tasks that need more flexibility or are cumbersome with nested methods.

Node retrieval is an integral part of TestFX and used in FxRobot and FxAssert.

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Added NodeQuery interface:

public interface NodeQuery {
    NodeQuery from(Node... parentNodes);
    NodeQuery from(Set<Node> parentNodes);

    NodeQuery lookup(String query);
    NodeQuery lookup(Predicate<Node> predicate);
    NodeQuery lookup(Matcher<Object> matcher);
    NodeQuery lookup(Function<Node, Set<Node>> function);

    NodeQuery select(Predicate<Node> predicate);
    NodeQuery select(Matcher<Object> matcher);
    NodeQuery selectAt(int index);

    @Nullable Node queryFirst();
    Optional<Node> tryQueryFirst();
    Set<Node> queryAll();
}

Updated NodeFinder interface:

public interface NodeFinder {
    NodeQuery nodes();
    NodeQuery nodes(String query);
    NodeQuery nodes(Predicate<Node> predicate);
    NodeQuery nodes(Matcher<Object> matcher);

    NodeQuery nodesFrom(Node... parentNodes);
    NodeQuery nodesFrom(Set<Node> parentNodes);

    Node rootNode(Window window);
    Node rootNode(Scene scene);
    Node rootNode(Node node);

    Node rootNode(Predicate<Window> predicate);
    Node rootNode(int windowIndex);
    Node rootNode(Pattern stageTitlePattern);
    Node rootNode(String stageTitle);
}

[hastebrot]

cc: #167