common: Introduced Accessor for traversing the scene-tree.

common: Introduced Accessor for traversing the scene-tree.

Basically, this Accessor is a utility to debug the Scene structure,
by traversing the scene-tree by users.

You can search specific nodes to read the property information,
figure out the structure of the scene tree and its size.

We actually don't recommend you to touch the property unless you really
know the each paint's position and role because it's not visible, difficult to
understand its anatomy.

Also, You must underatnd that modifying the nodes of the scene will be going
well with both the art-design structure and the prorgram logic.

In this first version, Accessor only supports for the Picture class.

@example:

auto picture = tvg::Picture::gen();
picture->load("test.svg");

//The callback function from lambda expression.
//This function will be called for every paint nodes of the tree.
auto f = [](const tvg::Paint* paint) -> bool
{
    if (paint->identifier() == Shape::identifier()) {
        //override properties?
        uint8_t r, g, b, a;
        paint->fillColor(&r, &g, &b, &a);
        paint->fill(r / 2, g / 2, b / 2, a);
    }

    //You can return false, to stop traversing immediately.
    return true;
};

auto accessor = tvg::Accessor::gen();

picture = accessor->access(move(picture), f);

...

@Issue: https://github.com/Samsung/thorvg/issues/693

Change-Id: I4f46754de46dfab9bb5b14f58cc659237344822a