removed TTextWriterEcho class, replaced by more generic TTextWriter.EchoAdd() and EchoRemove() methods;
also added TSynLogFamily's NoFile and EchoCustom properties to allow third-party logging - see feature request [91a114d2f6] -
and added MultiEventAdd(), MultiEventRemove() and MultiEventFind() functions, allowing easy implementation of a dynamic list of callbacks (just like C# delegates)