Construct Gobjects from java classes

[jwharm]

With this PR, Java-GI will internally keep a map of Java Class to GObject GType values. This results in two improvements:

1. When registering a new type (a Java class that is derived from GObject), it is not necessary anymore to keep the GType in a local field and expose it with a public static Type getType() method anymore.
2. When creating a new instance, it is now possible to call new GObject(Foo.class). No need to pass the GType (though the overload is still available).

As a result, there is less boilerplate code to write and maintain, when a Java class is registered in the GObject type system and new instances are constructed.

Before:

public class Foo extends GObject {
    private static Type gtype = Types.register(Foo.class);

    public static Type getType() {
        return type;
    }

    public Foo() {
        super(GObject.newInstance(Foo.getType());
    }

    public Foo(MemorySegment address) {
        super(address);
    }
}

After:

public class Foo extends GObject {
    static {
        Types.register(Foo.class);
    }

    public Foo() {
        super(GObject.newInstance(Foo.class);
    }

    public Foo(MemorySegment address) {
        super(address);
    }
}