Refactor StringManipulator tool so it works just with string and not work item field

[satano]

This should fix #2538

As mention id that bug report, custom user mapping is not applied during migration. @LudekStipal pointed to the code it was responsible for this. The problem is not the StringManipulatorTool itself, but the value which is used in that branch of code. The user mapping and all the other field mapping is done using oldWorkItem object, which is of type Field. But that specific branch for strings was processed using oldWorkItemData object which is of type FieldItem. So it is the different object and for user fields, it is not working with mapped user names, because the are mapped in the other place.

This is at least a bug for user fields which are mapped as @LudekStipal found. But I think, that is is also a potential source of bugs in the future, if something will be changing here. Because the one who will be changing something here may not notice, that populating work item fields it is done in different way for the strings.

StringManipulatorTool was working with FieldItem type. At first, I wanted to change it to work with Field type, so it would work the same way as user mapping. But this is not possible, because Field type is from TFS client library, but StringManipulatorTool in in different (lower level) assembly where this is not accessible. So I changes StringManipulatorTool that it does not take a whole field as argument, but just a string and returns new string. Basically, it does what it name suggests – takes a plain string, manipulates it and returns a new string. It was used only in this one place.

This may be a breaking change. The potential problem may be, than FieldItem was modified by StringManipulatorTool until now and now it remains the same. I am not sure if this can be a real issue.