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
private Image menuImage, disabledMenuImage = null;
	/**
	 * Font to be used for the info area text. Computed based on the dialog's
	 * font.
	 */
	private Font infoFont;
	
	/**
	 * Font to be used for the title area text. Computed based on the dialog's
	 * font.
	 */
	private Font titleFont;
	/**
	 * 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