Bug 118163 [Dialogs] - PopupDialog should define constants for zero margins
PopupDialogs traditionally use a 0 margin width and height for the grid layouts used. This should be made an API constant rather than subclasses just knowing.
bundles/org.eclipse.jface/src/org/eclipse/jface/dialogs/PopupDialog.java
* Flags indicating whether we are listening for shell deactivate events,
	 * either those or our parent's. Used to prevent closure when a menu command
	 * is chosen or a secondary popup is launched.
	 */
	private boolean listenToDeactivate;
	private boolean listenToParentDeactivate;
	/**
	 * Flag indicating whether focus should be taken when the dialog is opened.
	 */
	private boolean takeFocusOnOpen = false;
	/**
	 * Flag specifying whether a menu should be shown that allows the user to
	 * move and resize.
	 */
	private boolean showDialogMenu = false;
	/**
	 * Flag specifying whether a menu action allowing the user to choose whether
	 * the dialog bounds should be persisted is to be shown.
	 */
	private boolean showPersistAction = false;
	/**
	 * Flag specifying whether the bounds of the popup should be persisted. This
	 * flag is updated by a menu if the menu is shown.
	 */
	private boolean persistBounds = false;
	/**
	 * Text to be shown in an optional title area (on top).
	 */
	private String titleText;
	/**