singleton

Change-Id: If8311c27bcaba9dc0b3a0bf303ea31d7656b256b
Signed-off-by: Lars Vogel <Lars.Vogel@vogella.com>
Reviewed-on: https://git.eclipse.org/r/c/platform/eclipse.platform.ui/+/189516
Tested-by: Platform Bot <platform-bot@eclipse.org>
Reviewed-by: Karsten Thoms <karsten.thoms@karakun.com>