Set flow rate (extrude-mutliplier) for each extruder individually

The existing parameter 'extrudemultiply' sets a multiplier for all extruders. With an increasing number of dual extruders on Ultimaker Original machines, an individual setting for the multiplier has been requested (http://umforum.ultimaker.com/index.php?/topic/3580-flow-rate-adjustment-ulticontroller/). Indeed, there is some good reason to be able to change the flow for only one extruder during print.
This is a realisation of an ADDITIONAL multiplier for each extruder. For compatibility reasons I left the existing multiplier as it is. This includes the call up by the GCODE M221.
'M221 Sx' still changes the existing multiplier for all extruders.
'M221 Sx Ty' however changes the new individual multiplier for only one specified extruder.
As a consequence, new items have been added to the 'Tune' menu.
The code was successfully tested on an Ultimaker Original.