Blender Git Loki

Icons: Update blender_icons.svg to have a document background

When opening the SVG you wouldn't even see the icons, since they are all white
now. You'd always have to set up a document background color and make sure to
undo that again before saving the file, so it's not written into it.
Just make everyone's life easier by saving the file with a proper background
(gray with checkerboard).