[FIX] (zOrder) zOrder was not being taken into account when, in #add, a mark would be returned with a zOrder already specified.
[OPT] (Events Remove) Now explicitly removing event handlers from root elements, when a root panel is disposed (as when its canvas is stoken).
[OPT] (Root Panel Detached) Root SVG element is now fully built detached from the DOM, and only then attached.
[OPT] (Dom) Some code optimizatioins.
[OPT] (CanvasStolen) Some code optimizations by try/finally elimination and confinig to pv.Panel instead of pv.Mark.