Bug 847120: Move SVGFEOffsetElement to its own file r=Ms2ger

--HG--
rename : content/svg/content/src/nsSVGFilters.cpp => content/svg/content/src/SVGFEOffsetElement.cpp
rename : content/svg/content/src/nsSVGFilters.cpp => content/svg/content/src/SVGFEOffsetElement.h
Bug 847120: Convert SVGFEOffsetElement to WebIDL r=Ms2ger