removing @ from flowtree. (but cloning boxes when creating a display list)
auto merge of #1363 : ryanhc/servo/owned_layout, r=metajack

Removing all @ from flowtree to make it parallelism-ready.
The connection between displaylist and flowtree is removed, i.e., when displaylist is built, box is cloned.
Whether displaylist still needs to have boxes is a design decision to make.

Also thanks to @ksh8281