Fix notification badges position and size

Do not use absolute values for badge size and position,
instead use values based on SUGAR_SCALING.

Absolute values make notification icon badge look smaller
or bigger depending on SUGAR_SCALING value. This
also affects the badge position.

ie., the badge looks considerable smaller on a XO, and badge
position was also completely off, compared to sugar-build (and
the original design).

Signed-off-by: Martin Abente Lahaye <tch@sugarlabs.org>