GWT Backend: fixed generated getter/setter for primitive fields

This fixes the problem described in PR #1005 by strongly typing the generated getters/setters:

    // com.badlogic.gdx.math.Vector2#x
    private native float g1866(com.badlogic.gdx.math.Vector2 obj) /*-{return obj.@com.badlogic.gdx.math.Vector2::x;}-*/;
    private native void s1865(com.badlogic.gdx.math.Vector2 obj, float value)  /*-{obj.@com.badlogic.gdx.math.Vector2::x = value;}-*/;