Bug 181121 Model should provide utility method for cube to get correspond dimension key.
In model cube handle, the relation of cube data set and dimension data sets are managed by a series of dimension condition handles.We need model to add utility method so that we can easily get corresponding dimension data set column names according to given cube dimension keys.
model/org.eclipse.birt.report.model/src/org/eclipse/birt/report/model/core/Structure.java
target = newRef.getTargetElement( );
			if ( target != null && me != null && propName != null )
				target.addClient( me, propName );
		}
	}
	/**
	 * Returns the value of a structure property represented as a member
	 * variable.
	 * 
	 * @param propName
	 *            name of the property
	 * @return the value of the property, or null if the property is not set
	 */
	protected abstract Object getIntrinsicProperty( String propName );
	/**
	 * Sets the value of of a structure property represented as a member
	 * variable.
	 * 
	 * @param propName
	 *            the name of the property to set
	 * @param value
	 *            the property value
	 */
	protected abstract void setIntrinsicProperty( String propName, Object value );
	/**
	 * Validate whether this structure is valid. The derived class should
	 * override this method if semantic check is needed. The each error is the
	 * instance of <code>PropertyValueException</code>.
	 * 
	 * @param module