BUGFIX: Selection.BorderWidth was not having any effect.
CHANGE: To using screenX and mozInnerScreenX of documents, further changes in this direction needed.
CHANGE: Element is now a sibling of the browser (within the same xul:stack) and is back to using border rather than outline
CHANGE: All sub-documents are calculated at the time of indexing.
CHANGE: Removed some accessors and made them properties in favor of speed.