Fix notification badges position and size

Do not use unscaled magic numbers badge size and position,
as this caused badges to look and locate differently with
different SUGAR_SCALING values.

Instead, honor Icon class _BADGE_SIZE for calculating the
badge size.

Also, locate the badge at the bottom-left corner of the icon.

This PR requires https://github.com/sugarlabs/sugar-toolkit-gtk3/pull/136

Signed-off-by: Martin Abente Lahaye <tch@sugarlabs.org>