Changes to widget size and position code

Properties _base_x, _base_y, _base_width and _base_height store the
absolute pixel values of widget's position and size.
Properties _x, _y, _width and _height store the normalized (or
non-normalized if BGUI_NO_NORMALIZE is set)
Properties x, y, position, width, height and size are used to access _x,
_y, _width, _height.