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
	 */
	EReference getText_Font( );
	/**
	 * Returns the meta object for the containment reference '{@link org.eclipse.birt.chart.model.attribute.Text#getColor <em>Color</em>}'.
	 * <!-- begin-user-doc --> <!-- end-user-doc -->
	 * @return the meta object for the containment reference '<em>Color</em>'.
	 * @see org.eclipse.birt.chart.model.attribute.Text#getColor()
	 * @see #getText()
	 * @generated
	 */
	EReference getText_Color( );
	/**
	 * Returns the meta object for class '{@link org.eclipse.birt.chart.model.attribute.TextAlignment <em>Text Alignment</em>}'.
	 * <!-- begin-user-doc --> <!-- end-user-doc -->
	 * @return the meta object for class '<em>Text Alignment</em>'.
	 * @see org.eclipse.birt.chart.model.attribute.TextAlignment
	 * @generated
	 */
	EClass getTextAlignment( );
	/**
	 * Returns the meta object for the attribute '{@link org.eclipse.birt.chart.model.attribute.TextAlignment#getHorizontalAlignment <em>Horizontal Alignment</em>}'.
	 * <!-- begin-user-doc --> <!-- end-user-doc -->
	 * @return the meta object for the attribute '<em>Horizontal Alignment</em>'.
	 * @see org.eclipse.birt.chart.model.attribute.TextAlignment#getHorizontalAlignment()
	 * @see #getTextAlignment()
	 * @generated
	 */
	EAttribute getTextAlignment_HorizontalAlignment( );
	/**