Bug 322356 EPS's isPartVisible(MPart) method does not consider placeholders
SSIA
tests/org.eclipse.e4.ui.tests/src/org/eclipse/e4/ui/tests/application/HeadlessContextPresentationEngine.java
for (MWindow childWindow : perspective.getWindows()) {
					createGui(childWindow, element, perspective.getContext());
				}
			}
		} else if (element instanceof MPlaceholder) {
			MPlaceholder placeholder = (MPlaceholder) element;
			MUIElement ref = placeholder.getRef();
			if (ref != null) {
				createGui(ref);
				ref.setCurSharedRef(placeholder);
			}
		}
		return null;
	}
	/*
	 * (non-Javadoc)
	 * 
	 * @see
	 * org.eclipse.e4.ui.workbench.IPresentationEngine#createGui(org.eclipse
	 * .e4.ui.model.application.MUIElement)
	 */
	public Object createGui(MUIElement element) {
		return createGui(element, null, getParentContext(element));
	}
	public void removeGui(MUIElement element) {
		if (element instanceof MElementContainer<?>) {
			for (Object child : ((MElementContainer<?>) element).getChildren()) {
				if (child instanceof MUIElement) {
					removeGui((MUIElement) child);
				}