Small !CATEGORY update

[Travis Cobbs]

FYI, unless there is a bug in my script that I use to generate the PDF parts catalog, the following categories are not used by any official parts:

• Forklift
  
• Grab
  
• Helper
  
• Lever
  

Since Helper is new, that's I assume not unexpected. We might want to consider whether the others should be left as valid categories, though.

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.)