Bug 454897: RAP: Fix that calling DefaultShellService.shellOpen will
lead to an exception if not in the model thread.

Change-Id: I98abbf7e69b51cc4befc6f4c3e6f5eab279dc72e
Signed-off-by: Patrick Bänziger <patrick.baenziger@bsiag.com>
Task-Url: https://bugs.eclipse.org/bugs/show_bug.cgi?id=454897
Reviewed-on: https://git.eclipse.org/r/38049
Tested-by: Hudson CI
Reviewed-by: Claudio Guglielmo <claudio.guglielmo@bsiag.com>