Bug 183919 [Enhancement] add a switch to determine whether to clean viewer image folder when session expires
in current viewer implementation, file in image folder is deleted when its corresponding session expires, could viewer add a switch to allow users to decide whether this deletion operation should be performed.
viewer/org.eclipse.birt.report.viewer/birt/WEB-INF/classes/org/eclipse/birt/report/utility/ParameterAccessor.java
String realPath = null;
		boolean isRelative = false;
		path = convertSystemPath( path );
		
		if ( path == null || path.trim( ).length( ) <= 0 )
		{
			path = DataUtil.trimString( defaultPath );
		}
		
		if ( isRelativePath( path ) )
		{
			isRelative = true;
			if ( !path.startsWith( "/" ) ) 
				path = "/" + path; 
			realPath = DataUtil.trimSepEnd( getRealPath( path, context ) );
		}
		else
		{
			
			realPath = DataUtil.trimSepEnd( path );
		}
		boolean flag = makeDir( realPath );
		
		if ( !canWrite )
			return realPath;
		
		if ( flag )
		{
			try
			{