Removed `isBeingReset()`, exposed variable `m_ongoing_reset` and renamed to `ar_`

The problem was introduced by myself in 9431e1b - I cleaned up duplicate syncing code but oversimplified the logic to reset unlinked actors. As result, the physics paused state and skeletonview state could only be active on player's actor and connected actor, not on any free standing one.

This is quite a significant remake of actor-linking code; a direct follow-up to d36c4ec which introduced ✉️ MSG_SIM_ACTOR_LINKING_REQUESTED. A remake was needed because there was no single spot to correctly reset the skeleton/physicspause on actor unlink - there was simply no concept of "actors were just linked/unlinked". While researching how to add it, I realized scripts may want to know when that happens, so I added a script event `SE_GENERIC_TRUCK_LINKING_CHANGED` for it. Keep in mind there can be multiple interlinking beams at the same time, so not every added/removed beam means linking changes. The new event also helps navigate the codebase and documents how stuff works.

Note that only code changed; all data remained the same - code handling hooks/ties/ropes/slidenodes got only cosmetic edits.

Code changes:
* ScriptEvents.h - added SE_GENERIC_TRUCK_LINKING_CHANGED ~ args: #1 action (1=linked, 0=unlinked), #2 link type `ActorLinkingRequestType` (will be INVALID on savegame load or forced unlink when deleting actor) #3 master ActorInstanceID_t, #4 slave ActorInstanceID_t
* Actor.h/cpp - Removed functions `Add/RemoveInterActorbeam()` - using new ones in ActorManager.
* ActorManager.h/cpp -  Added newly implemented functions `Add/RemoveInterActorBeam()`.

💡 This introduces the mechanism of grouping beams in `Actor::ar_beams` by their type and origin. See `struct BeamRangesByOrigin` in file 'SimData.h'.

Problem `a` and `b` were just references-to-smartpointer, not full smartpointers, the actual data lived in `inter_actor_links` - we deleted that data and then strill tried using `a`.

Additional fix: faulty assert and debug check (operators were swapped).