fix(gameObject): move / remove dom nodes when add / remove children

[straker]

Decided not to make the property public and instead make the GameObject addChild and removeChild functions do the moving. That way I don't need to remove the HTML obfuscation.

Closes #365

[straker]

This won't work as expected if creating complex DOM setups, like a table or grid, where there's an intermediate HTML node like a tr or div. For those case the DOM node really needs to be public to append it to the correct HTML element (and not the GameObject element)