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
* @see org.eclipse.birt.chart.model.attribute.Stretch
	 * @see org.eclipse.birt.chart.model.attribute.impl.AttributePackageImpl#getStretch()
	 * @generated
	 */
	int STRETCH = 62;
	/**
	 * The meta object id for the '{@link org.eclipse.birt.chart.model.attribute.StyledComponent <em>Styled Component</em>}' enum.
	 * <!-- begin-user-doc --> <!-- end-user-doc -->
	 * @see org.eclipse.birt.chart.model.attribute.StyledComponent
	 * @see org.eclipse.birt.chart.model.attribute.impl.AttributePackageImpl#getStyledComponent()
	 * @generated
	 */
	int STYLED_COMPONENT = 63;
	/**
	 * The meta object id for the '{@link org.eclipse.birt.chart.model.attribute.TickStyle <em>Tick Style</em>}' enum.
	 * <!-- begin-user-doc --> <!-- end-user-doc -->
	 * @see org.eclipse.birt.chart.model.attribute.TickStyle
	 * @see org.eclipse.birt.chart.model.attribute.impl.AttributePackageImpl#getTickStyle()
	 * @generated
	 */
	int TICK_STYLE = 64;
	/**
	 * The meta object id for the '{@link org.eclipse.birt.chart.model.attribute.TriggerCondition <em>Trigger Condition</em>}' enum.
	 * <!-- begin-user-doc --> <!-- end-user-doc -->
	 * @see org.eclipse.birt.chart.model.attribute.TriggerCondition
	 * @see org.eclipse.birt.chart.model.attribute.impl.AttributePackageImpl#getTriggerCondition()
	 * @generated
	 */
	int TRIGGER_CONDITION = 65;
	/**