Small !CATEGORY update

[Orion Pobursky]

I almost forgot: POV-Ray and String are used as category by at least one official part each. We obviously don't want POV-Ray (used for light.dat), but String should perhaps be added to the list. (It seems at least conceivable that LEGO would have a string with different thickness at some point.)

An update to light.dat (incidentally that last part with release ORIGINAL) is on the PT. I suppose it can use the Helper category but moving it to the /h folder isn't possible just yet.

I agree with String and will add it.