Adapt close-hovering icon for notification API

The close icon for showing notifications has been adapted with a more
modern look and feel. The "active" version of the icon, used when
hovering over it, is currently still the old one and does not fit the
new icon anymore.

This change introduces a new "active" version of the notification close
icon, being a thicker version of the non-active one.

Fixes #2043

Author: HeikoKlare