Bug 950174: Part 1: Move required session store files from browser/ to toolkit/modules. r=gps, ttaubert

--HG--
rename : browser/components/sessionstore/src/FormData.jsm => toolkit/modules/sessionstore/FormData.jsm
rename : browser/components/sessionstore/src/ScrollPosition.jsm => toolkit/modules/sessionstore/ScrollPosition.jsm
rename : browser/components/sessionstore/src/XPathGenerator.jsm => toolkit/modules/sessionstore/XPathGenerator.jsm
Bug 950174: Part 2: Form data and scroll position save and restore. r=mbrubeck