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
if ( context != null )
			return context.element;
		return null;
	}
	/**
	 * Gets the element property name in the cached context.
	 * 
	 * @return the property name
	 */
	public String getContextPropertyName( )
	{
		if ( context != null )
			return context.elementPropName;
		return null;
	}
	/**
	 * Caches the context to the structure.
	 * 
	 * @param context
	 *            the context
	 */
	public void setContext( StructureContext context )
	{
		this.context = context;
	}
	/**
	 * The structure context. It is used when establishes back reference.
	 * 
	 */
	public static class StructureContext