containments/desktop: only bind AppletsLayout if containment

The desktop containment is used both for the desktop and for other FolderViews on the panel or desktop. Only the full desktop folderview can contain further applets; panel applets or extra folder views on the desktop are not containments. However, they still try to set themselves up with a ContainmentLayoutManager.AppletsLayout, which fails with a warning because they are not containments.

Instead, only set the AppletsLayout containment property if it's actually a containment. This is effectively what happens anyway if the assignment fails, but skips the warning.