(fix) FX units not working on all samplers

[ronso0]

Fixes #15799

• new profile (or just no effects.xml and effects dir)
• load a track into sampler 5, play
• assign FX1, load Echo etc., turn all on and turn Meta so there's some good feedback/filter effect
• close, restart
• enable Echo, press Play on sampler 5
  -> this PR: enjoy the noise : )
  -> 2.5: no echo. toggle Fx1 Off and On before engaging Echo & Co, then it works.

The root cause:
short: the fx I/O routing table is not updated instantly when additional samplers are added to the engine (or any inputs actually).

long:

• CoreServices adds the default inputs (4 decks, 4 samplers, mics, aux etc.) and upon creation they are registered in EffectsManager
• CoreServices then calls m_pEffectsManager->setup() which initializes all EngineEffectChains with the registred inputs/outputs
• then samplers.xml is parsed and maybe more samplers are created -- same when a sampler bank is loaded manually or when we switch to a skin with more samplers
• new inputs are registered in EffectsManager, then in each (standard) effect chain and the Fx routing CO states are also restored correctly (see GUI, FX1 on sampler 5)
  -> BUT the I/O maps used by EngineEffectChains are not updated, hence engaging an effect has no ..effect for those new inputs
  -> the I/O maps are extended for the new key, but ChannelHandleMap::maybeExpand abviously can't initialize the outputs map

The fix:

• store the outputs map when creating EngineEffectChain(it won't change as no new outputs are added after EffectsManager::setup()-- afaict)
• when registering and enabling a new input, use ChannelHandleMap[input] as before but then assign the stored output map

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Maybe alternatively we could add a method EngineEffectChain::registerInputChannel and use a new EffectsRequest::MessageType to send the request via the message pipe.
But I doubt it has any benefits over the current fix. Does it?