Dev branch code cleanup (macOS/Linux)

[Packetdancer]

General cleanup to get everything compiling on macOS and Linux, as well as making a couple of things const.

This includes the cleanup patch against main as well, just for the sake of everyone's sanity.

[Packetdancer]

Details on individual changes:

• Cleaned up exports, minor details that MSVC lets skate but which the far-stricter clang dislikes, etc.
• Changed UInputAction to const (or TObjectPtr around a const) in a number of places, to allow things to be bound to multiple UInputActions; taking the SourceAction from an input action instance returns a const version, necessitating this change.
• Made it possible to override IsPeriodPaused in effects, to allow for more complex pause/unpause logic in C++ implementations.

[Packetdancer]

Discovered on my lunch break today that UE 5.4 seems to use some stricter compile settings on macOS and Linux (or else I changed what UE 5.3 does on my installs and forgot), so I made a few more changes to deal with nitpicky clang warnings/errors.

[reznok]

Thanks!