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/context/ViewerAttributeBean.java
+ "_" + IBirtConstants.PROP_TYPE; 
							ConfigVariable typeVar = handle
									.findConfigVariable( typeVarName );
							
							String dataType = ParameterDataTypeConverter
									.ConvertDataType( parameter.getDataType( ) );
							String cachedDateType = null;
							if ( typeVar != null )
								cachedDateType = typeVar.getValue( );
							
							if ( cachedDateType == null
									|| !cachedDateType
											.equalsIgnoreCase( dataType ) )
								continue;
							
							String exprVarName = configVar.getName( ) + "_" 
									+ IBirtConstants.PROP_EXPR;
							ConfigVariable exprVar = handle
									.findConfigVariable( exprVarName );
							String cachedExpr = null;
							if ( exprVar != null )
								cachedExpr = exprVar.getValue( );
							if ( cachedExpr == null )
								cachedExpr = ""; 
							String expr = parameter.getValueExpr( );
							if ( expr == null )
								expr = ""; 
							
							if ( !cachedExpr.equals( expr ) )
								continue;