Update Notification API to use modern looking X

The find/replace overlay introduces a new close icon similar to the
close icon used for the tab folder from
https://bugs.eclipse.org/bugs/show_bug.cgi?id=575395

This pull request updates the notification close icon which still looks
like the old tab icon before the change in
https://bugs.eclipse.org/bugs/show_bug.cgi?id=575395  and does not fit
anymore in the overall look and feel.

A follow-up PR could also update the active folder but find a new
"reddisch" color would require more testing on the different OS, while
with the regular icon we can build on the testing of
#1991

Fixes: #2035

Author: vogella