automatically hide the slices layer when rendering

[Lee Braiden]

Currently, SVGSlice requires you to manually hide the slices layer before exporting. This would ideally happen automagically. It'll involve reading the whole XML file, modifying some attributes, writing it out to a temporary file, and then doing all the other SVGSlice stuff as normal on the new file. Shouldn't be a major task, but will take a little time.