The FindReplaceOverlay currently expects a target control that is an
instance of Scrollable to provide a horizontal and vertical scrollbar.
This is invalid, as per API the according getter methods do not
necessarily return scrollbars. In consequence, exceptions occur that
lead to an improperly positioned overlay, as the update callback for the
position and size aborts.

This change ensures that scrollbars of the target control are only
considered when available.

Fixes https://github.com/eclipse-platform/eclipse.platform.ui/issues/2147