Bug 275910 [Widgets] PaintListener on Tree not invoked when item count is 0
bundles/org.eclipse.ui.ide/src/org/eclipse/ui/internal/views/markers/ExtendedMarkersView.java
* Return the group used for categorisation.
	 * 
	 * @return MarkerGroup
	 */
	MarkerGroup getCategoryGroup() {
		return builder.getCategoryGroup();
	}
	/**
	 * Return the clipboard for the receiver.
	 * 
	 * @return Clipboard
	 */
	Clipboard getClipboard() {
		if (clipboard == null)
			clipboard = new Clipboard(viewer.getControl().getDisplay());
		return clipboard;
	}
	/**
	 * Return the content provider for the receiver.
	 * 
	 * @return ITreeContentProvider
	 * 
	 */
	private IContentProvider getContentProvider() {
		return new MarkerViewerContentProvider(this);
	}
	/**
	 * Get the id of the marker field in treeColumn.
	 * 
	 * @param treeColumn