Added new methods is/setStatusVisible to AbstractFormField, required for
Html UI.

Change-Id: Ie01e249d8c41e542ff16aeff659e875c096ba775
Signed-off-by: André Wegmüller <awe@bsiag.com>
Reviewed-on: https://git.eclipse.org/r/38416
Tested-by: Hudson CI
Reviewed-by: Claudio Guglielmo <claudio.guglielmo@bsiag.com>