Add newer checkbox images

These have been created from scratch in Inkscape.

Please let me know if you would like the SVG in this
directory, too, and I will put it there.

Signed-off-by: Cruz Julian Bishop <cruzjbishop@gmail.com>
Merge branch 'new-checkboxes' of github.com:CruzBishop/frontend into CruzBishop-new-checkboxes