Bug 213671 Expose evaluate() method in IReportContext
Need Engine expose evaluate() method in IReportContext interface, this will be used to evaluate global Javascript expression in case there's no resultset bound to current report item.
engine/org.eclipse.birt.report.engine/src/org/eclipse/birt/report/engine/api/script/IReportContext.java
/**
	 * Add the object to report document scope. This object can be retrieved
	 * later. It is persisted between phases, i.e. between generation and
	 * presentation.
	 */
	void setPersistentGlobalVariable( String name, Serializable obj );
	/**
	 * Remove an object from report document scope.
	 */
	void deletePersistentGlobalVariable( String name );
	/**
	 * Retireve an object from report document scope.
	 */
	Object getPersistentGlobalVariable( String name );
	/**
	 * Finds user-defined messages for the current thread's locale.
	 * 
	 * @param key
	 *            resource key of the user-defined message.
	 * @return the corresponding locale-dependent messages. Return
	 *         <code>null</code> if resoueceKey is blank.
	 */
	String getMessage( String key );
	/**
	 * Finds user-defined messages for the given locale.
	 * <p>
	 * First we look up in the report itself, then look into the referenced
	 * message file. Each search uses a reduced form of Java locale-driven
	 * search algorithm: Language&Country, language, default.
	 *