desktoppackage/WidgetExplorer: avoid scrollbar binding loop

The scroll bar visibility can cause a binding loop as it becoming visible can change the word wrap of the contents, which may causes the size to increase, which triggers re-evaluation of the visibility. This is jsut a temporary problem, as the size can't shrink due to the scrollbar becoming visible, and there's only one vertical scrollbar.

Instead, use a delayed binding to give it some extra time. This is similar to how this was handled in the notes applet.