Bug 119340 [PopupDialog] - need constant for horizontal spacing
PopupDialog provides a constant for vertical spacing used in the contents, but not for horizontal spacing.
bundles/org.eclipse.jface/src/org/eclipse/jface/dialogs/PopupDialog.java
* at the bottom of the dialog. Subclasses may override. Default behavior is
	 * to have an info area if info text was provided at the time of creation.
	 * 
	 * @return <code>true</code> if a title area should be created,
	 *         <code>false</code> if it should not.
	 */
	protected boolean hasInfoArea() {
		return infoText != null;
	}
	/**
	 * Creates the title and menu area. Subclasses typically need not override
	 * this method, but instead should use the constructor parameters
	 * <code>showDialogMenu</code> and <code>showPersistAction</code> to
	 * indicate whether a menu should be shown, and
	 * <code>createTitleControl</code> to to customize the presentation of the
	 * title.
	 * 
	 * <p>
	 * If this method is overridden, the returned control's layout data must be
	 * an instance of <code>GridData</code>. This method must not modify the
	 * parent's layout.
	 * 
	 * @param parent
	 *            The parent composite.
	 * @return The Control representing the title and menu area.
	 */
	protected Control createTitleMenuArea(Composite parent) {
		Composite titleAreaComposite = new Composite(parent, SWT.NONE);
		GridLayout layout = new GridLayout(2, false);
		layout.marginHeight = POPUP_MARGINHEIGHT;