Introduce a createText function.
Issue #852 - Implement the Node.textContent setter.
Pass the AbstractNode into the Node.textContent setter.
Make all DOM manipulation wait until it's safe to do so (ie. all reflows for the page have completed). Fix a race where a newly-initiated reflow would be considered complete when receiving the completion notice for the previous reflow.
Add a test for Node.textContent.
auto merge of #947 : jdm/servo/textcontent-setter2, r=metajack