Base class collapsing to work around hammer limitations

[vrad-exe]

Hammer has some annoying limitations on keyvalues inherited from base classes:

• Keyvalues from the base class can be overridden, but only with another of the same type - 90% of the cases where this would be useful are to expose it as a different type (e.g. turning a generic type into choices), but this is impossible
• It's not possible to remove a keyvalue defined in a base class, which is sometimes necessary (e.g. to work around the func_breakable_surf origin bug), or it'd just be nice to be able to hide things that don't affect a specific entity

It would be really useful to have these restrictions removed, and since we have the unifying FGD system, I can see a relatively straightforward way to do that - collapse/"inline" base classes directly into entities which make use of these abilities, so that they bypass Hammer's restrictions on base classes. For removing/hiding keys there would also need to be a new syntax added for that, which could likely look similar to the existing @resources blocks (e.g. @hidekey origin).

To keep file sizes down, this should only be applied to entities that actually need it.

[TeamSpen210]

Originally, I did actually have HA output completely collapsed FGD files, but I found problems there - I think VBSP broke or something, maybe hitting some limit. So I'm not sure how viable this is, if it accidentally affects too many entities. Instead of collapsing, maybe I should have it generate new copies of the base class hierarchy with the keys removed. Reuse those where possible, would work well.

[vrad-exe]

One important thing is that the original base classes should still be fully retained with their original names in the resulting FGD file, because someone might add a second FGD on top of the existing one which couldn't easily take that into account

[TeamSpen210]

Probably a good idea yeah.