common: introduce iterator

common: introduce iterator

+++

auto picture = tvg::Picture::gen();

auto func = [](const tvg::Paint* paint, const tvg::Paint* parent, bool hasChildren) -> int
{
    if (paint->identifier() == Shape::identifier())
        //TODO: override properties.

    //return true to continue, return false to stop.
    return true;
};

picture = tvg::Iteratorv::iterate(move(picture), func);

Change-Id: I2ea956cb46b20547239686969c2e403e23fc8943