Tweaks to support mod/graphics merging

[PeridexisErrant]

This PyLNP issue covers the background and rationale for this request. In short: mod authors wish to extend graphics packs, and merging TwbT overrides is the largest pain point.

Fortunately this is solvable with the following requested changes, which I and they hope will be reasonably easy to implement:

• Support multiple overrides files, to prevent clobbering. (I'd suggest following the example of DFHack init files and globbing for overrides*.txt)
• Identifying item subtypes by name for overrides, to prevent problems with mods that may add or remove subtypes.

Personally, I'd be very happy if the use of numbers to identify a thing-to-override was deprecated, and at least printed a warning to the console - otherwise nigh-undiagnosable errors may still creep in.

Wholesale removal of numeric identifiers or moving to the file-per-sprite system you mentioned a while ago would also be lovely, but seem further off - and I'd rather go for the faster and easier request.

On behalf of the DFgraphics group - thanks.

[PeridexisErrant]

Multiple overrides files has been proposed before as #44. This is distinct from #5, as it is often impossible to know in advance what overrides file(s) exist.