443421: MessageBox shows with big delay 

Change-Id: Ie6dc9f2467a60221539a116ac9f137ca3fc8c552
Task-Url: https://bugs.eclipse.org/bugs/show_bug.cgi?id=443421
HtmlUi: added method isOutlineChanging() to IDesktop (required for Html
UI).

Change-Id: I4cdadb8c69dab7a989c267c6d0c10ec90001c195
Signed-off-by: André Wegmüller <awe@bsiag.com>
Reviewed-on: https://git.eclipse.org/r/38360
Tested-by: Hudson CI
Reviewed-by: Claudio Guglielmo <claudio.guglielmo@bsiag.com>