Add a reference to the currently used tool to SpindleMonitoringType

[ChrisDahmen]

Currently the only way I see to display a relation between the Spindle and the Tool is to adapt the location of a tool to be "Spindle", which is not quite right in it's own way, since the toolholder in the magazine should still remain assigned to that tool, for when it needs to be swapped out again.
It would be nice if the SpindleMonitoringType would get a Variable, which references the active tool in that spindle in some way, for example by containing the NodeId of the ToolType or by referencing the ControlIdentifier1 and ControlIdentifier2 variables present in tools.