Cherry pick upstream changes

[braem]

Tackles part of https://github.com/ChaosInitiative/Chaos-FGD/issues/19

Cherry picks the bulk of the changes from teamspen's repo up until now.

- Stuff I didn't pull in -

https://github.com/TeamSpen210/HammerAddons/commit/cd7264dd438d63e1a6a78c9a4289077d1dece0e8 - dont have this in engine

https://github.com/TeamSpen210/HammerAddons/commit/084c7829aee3d21e60e1a79ad14b12d56ccef31a - added our own Brightness/SetBrightness field/input as using renderamt requires spotlight to be recreated (cant change dynamically).

https://github.com/TeamSpen210/HammerAddons/commit/c10e00af3632466219df08897ec4423df1f05fb4 , https://github.com/TeamSpen210/HammerAddons/commit/6543cef4ebad32f99d56bbadefbcc9f3212fe8f9 - extension KV stuff. Deserves it's own PR if we want it.

https://github.com/TeamSpen210/HammerAddons/commit/04897842673ef4f3bf6d41d647ffc047985a7a4e , https://github.com/TeamSpen210/HammerAddons/commit/7f2850789301fc26a7232bf335331d70588c0661 - clarifying visgroups. will need it's own PR (might not be able to cherrypick with all our changes)

https://github.com/TeamSpen210/HammerAddons/commit/bd308fcb7303fd13657bd8d24b5a5174fef15f50 - have this already (thx goc)

https://github.com/TeamSpen210/HammerAddons/commit/76bfcb584163f5535531098adb71b7b24f34c90a - bye bye min/max dxlevel

https://github.com/TeamSpen210/HammerAddons/commit/5759c7004acc43988a76b2f25f835a3886fdc9b9 - this mistake caused the entity to be missed in our big removal PRs. It's not in our engine so I removed it.

https://github.com/TeamSpen210/HammerAddons/commit/a795341192997ed209d884de4460d31112051295 - wondering what this is for

https://github.com/TeamSpen210/HammerAddons/commit/df576061087637af48eba87952701302adb4e623 , https://github.com/TeamSpen210/HammerAddons/commit/84b6401f9b1d6792079e21476c245bbc6c37810c , https://github.com/TeamSpen210/HammerAddons/commit/14e6df4bb5fb3748426388f29f8d64b9d5530903 - material exclusions, could be useful? Deserves it's own PR

https://github.com/TeamSpen210/HammerAddons/commit/40649e10ef11209247fabc424bb5cdc215fd7a05 , https://github.com/TeamSpen210/HammerAddons/commit/2296e1f7306c87e9047e0c36e3a8730be2b54f52 , https://github.com/TeamSpen210/HammerAddons/commit/69b7137e77810918231076146512e07c2808b3bb , https://github.com/TeamSpen210/HammerAddons/commit/a52c6a1e9246b9b194e7ee2acfefbd8153482849 , https://github.com/TeamSpen210/HammerAddons/commit/bc572cc1e0995d7adfb7912ff62388e3710c2338 - addition of base entity. Will need to be it's own PR as could be more involved.

https://github.com/TeamSpen210/HammerAddons/commit/041232ddcc6594706036784983d2075d5c8a1623 , https://github.com/TeamSpen210/HammerAddons/commit/48b7055bf0f033c1261fc0d00d6b0b45d04be79d , https://github.com/TeamSpen210/HammerAddons/commit/cdc0f24119dfbe9b0badd1de55a67826d0e10426 , https://github.com/TeamSpen210/HammerAddons/commit/6d43f18a1a936c7dc4cfafb6e6f81f53f9a8ce46 , https://github.com/TeamSpen210/HammerAddons/commit/49d26ccee6274696f7256bf81a636c5f0c4a16c1 , https://github.com/TeamSpen210/HammerAddons/commit/e233843457bd8f4b1bd3b3db68218615ddc96091 , https://github.com/TeamSpen210/HammerAddons/commit/dc8b1ca21818ef81780ca93d19fa0bf465db87dc , https://github.com/TeamSpen210/HammerAddons/commit/d6ab8c674d5eda03b4021134c851aad82f2b8a95 , https://github.com/TeamSpen210/HammerAddons/commit/13cbe6320da1552474b2c8d2e9ba2336c9a28d8b , https://github.com/TeamSpen210/HammerAddons/commit/8283234e2449ac10ac163c3390e8e3df7887c9c4 , https://github.com/TeamSpen210/HammerAddons/commit/7bbc3d9635ee2cfe556b3b206b24c1d787a680de - all L4D stuff; dont need

https://github.com/TeamSpen210/HammerAddons/commit/0eaaefed370aeb3399f176d903dd02cf19252acc , https://github.com/TeamSpen210/HammerAddons/commit/aa2e5089a88c4b814e0739404744480217a7adde , https://github.com/TeamSpen210/HammerAddons/commit/e0196d4dca887326a1d8d67023046712b9d3782d , https://github.com/TeamSpen210/HammerAddons/commit/d0b07b66a84758aadbec6dfc3fa112249e1e9f70 - readme changes. Should be in it's own PR as we will be changing the readme anyhow to be better inline with what Chaos is using this repo for (different goal than teamspen).

[TeamSpen210]

The modified Crowbar is used for propcombine, allowing it to automatically decompile the required props directly. The KV extensions I did is to prepare for implementing instance collapsing in srctools - it uses this database also to know how to transform the keys.