Bug 104986 Backgound image uses an absolute path
Currently Chart uses an absolute path to indicate a backgournd image, that may cause the design unusable on other environments. Expected the background image can be definded in multiple ways like in report designer. e.g. URI(absolute path), File(relative path), Embedded Image(Encoded Image Data).
chart/org.eclipse.birt.chart.engine/src/org/eclipse/birt/chart/model/attribute/AttributePackage.java
* @generated
	 */
	int GROUPING_UNIT_TYPE = 47;
	/**
	 * The meta object id for the '{@link org.eclipse.birt.chart.model.attribute.HorizontalAlignment <em>Horizontal Alignment</em>}' enum.
	 * <!-- begin-user-doc --> <!-- end-user-doc -->
	 * @see org.eclipse.birt.chart.model.attribute.HorizontalAlignment
	 * @see org.eclipse.birt.chart.model.attribute.impl.AttributePackageImpl#getHorizontalAlignment()
	 * @generated
	 */
	int HORIZONTAL_ALIGNMENT = 48;
	/**
	 * The meta object id for the '{@link org.eclipse.birt.chart.model.attribute.IntersectionType <em>Intersection Type</em>}' enum.
	 * <!-- begin-user-doc --> <!-- end-user-doc -->
	 * @see org.eclipse.birt.chart.model.attribute.IntersectionType
	 * @see org.eclipse.birt.chart.model.attribute.impl.AttributePackageImpl#getIntersectionType()
	 * @generated
	 */
	int INTERSECTION_TYPE = 49;
	/**
	 * The meta object id for the '{@link org.eclipse.birt.chart.model.attribute.LeaderLineStyle <em>Leader Line Style</em>}' enum.
	 * <!-- begin-user-doc --> <!-- end-user-doc -->
	 * @see org.eclipse.birt.chart.model.attribute.LeaderLineStyle
	 * @see org.eclipse.birt.chart.model.attribute.impl.AttributePackageImpl#getLeaderLineStyle()
	 * @generated
	 */
	int LEADER_LINE_STYLE = 50;
	/**
	 * The meta object id for the '{@link org.eclipse.birt.chart.model.attribute.LegendBehaviorType <em>Legend Behavior Type</em>}' enum.
	 * <!-- begin-user-doc -->