Expose collapsing state in render hook

[dmackdev]

Closes #29.

• Removes Copy/Clone derives for RenderContext structs.
• Exposes CollapsingState on property and expandable delimiter RenderContext structs so the render hook can manipulate/check the expanded state of arrays/objects.
• Updates JSON Editor UI demo to automatically expand arrays/objects when a new element is added to them, and disables the toggle buttons when an element is being edited.