Annotation to define GObject properties

[jwharm]

This adds a new @Property annotation that can be used with getter and setter methods in Java code to define GObject properties.

Example usage:

@Property(name = "n-items", type = ParamSpecInt.class, writable = false)
public int getNItems() {
    return items.size();
}

The property definitions in the ListIndexModel utility class now use this annotation.