Filter categories, and sort the parts within them.
Refactor: make LdrawVisitor generic
Refactor (part 2): make the visitor return an object of the generic type (existing visitors return void.)
Use a visitor to calculate bounds, accurately accounting for transforms.
update the parts used list without making it collapse.
don't show the annoying tooltip when dragging from set parts.
don't show the annoying tooltip when dragging from the main parts list either.