[Review request] Add support for pistols chambered in .45 ACP by introducing new model

[macinsight]

This PR relates to #39, adding support for differentiating calibers, specifically the .45 ACP caliber.

[macinsight]

@coavins It appears there's a slight issue (Check it yourself, changed the Glock 21 to the new Pistol_45acp model) but it's not jumping out at me. A second set of eyes would be appreciated.
Once this PR is merged, I'd like to update the docs to expand upon adding new models for the future.

[coavins]

Looks good to me - don't forget to add the item names into the translation files (English only is fine). They are under coavinsfirearms/media/lua/shared/Translate/EN. The PistolReceiver_45acp for example should probably be called "Pistol Frame (45 acp)" or something like that.

When disassembling a Glock 21, the name ("Glock 21 Pistol") will be prepended to the name of the receiver, so you will get something like "Glock 21 Pistol Pistol Frame (45acp)" depending on what name you use for the receiver. This is unfortunate, but we definitely want to make sure the receiver is identifiable as a pistol frame in the event that the player finds it on its own, without the name prepended like that.

Maybe we could improve that concatenation to avoid duplicate terms in cases like this.

Don't forget also that you can re-use parts between different models, so you could re-use the same pistol receiver and simply have a 45 acp barrel, if you're so inclined. Then players could swap the caliber just by finding a new barrel. But personally I would err on the side of realism.