Add logic to support nested Svgs

[marlenecota]

• Move the following out of RenderableView and into a new interface, IRenderable, that both RenderableView and SvgView implement:
  • SvgParent
  • Geometry
  • Render()
  • UpdateProperties()
  • MergeProperties()
  • SaveDefinition()
  • Unload()
• Switch GroupView's Children and SvgView's Templates from RenderableViews to IRenderables.
• Fix SvgRoot logic to check if an SvgView has an SvgParent as it is null in the root Svg element.
• If SvgView is inhering props from a parent, it passes those through to its Group.
• When setting an SvgView's SvgParent, we revoke event handlers and null out the canvas.