Instead of extracting item sheets one at a time, it's now done on
multiple threads. Loading gear is now faster since reused files are
cached, switching between races is still wasteful, but it's a good
enough improvement for now.
These parts (inspired by the KDE parts system) will allow the tooling
to reuse GUI mechanisms. Right now the two supported parts are for Excel
and Models, and exdviewer and mdlviewer will be retrofitted to them in
future commits.
