Render Charts in div

[boygirl]

this PR changes how portals are rendered to give z-index control to portal components

fixes FormidableLabs/victory#752

This change should also make it easy to create a custom html portal for rendering html labels and tooltips

This may be a breaking change for parent styles, as it changes the rendered tree

before:

<svg {...parentProps} overflow="visible">
  <svg {...parentProps}>
    {children}
  </svg>
    {title ? <title id={this.getIdForElement("title")}>{title}</title> : null}
    {desc ? <desc id={this.getIdForElement("desc")}>{desc}</desc> : null}
   // portalComponent renders a <g>
  {React.cloneElement(portalComponent, { ref: this.savePortalRef })}
</svg>

after:

<div style={defaults({}, style, divStyle)} className={className}>
  <svg {...svgProps} style={{ ...svgStyle, pointerEvents: "all" }}>
    {title ? <title id={this.getIdForElement("title")}>{title}</title> : null}
    {desc ? <desc id={this.getIdForElement("desc")}>{desc}</desc> : null}
    {children}
  </svg>
  <div style={{ ...divStyle, zIndex: portalZIndex, position: "relative", marginTop }}>
    // portalComponent renders an <svg>
    {React.cloneElement(portalComponent, { ...portalProps, ref: this.savePortalRef })}
  </div>
</div>