never let a new job pop up above an already shown one.

Change-Id: Ife1d803ceaae857f199179fbfd4b255c6843778d
Signed-off-by: Joerg Kubitz <jkubitz-eclipse@gmx.de>
Reviewed-on: https://git.eclipse.org/r/c/platform/eclipse.platform.ui/+/190578
Tested-by: Platform Bot <platform-bot@eclipse.org>