no Quick Effect loaded for decks 3 & 4 by default

[ronso0]

With a fresh install there are no Quick Effects loaded for decks 3 & 4.

• start Mixxx the first time (LateNight skin)
• Filter is loaded for decks 1/2
• toggle 4 decks: no effects for decks 3/4
• close, restart
• all decks have Filter loaded

I think the default should be Filter for all decks, without a need to configure anything.

The issue becomes obvious (no EQs on decks 3/4) with #11695 which also stores the EQ effects in effects.xml

I'm not sure whether that was always the case. I think it may have been the EQ Preferences which (unintentionally) have set the default for all decks before #11527.

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EffectsManager::readEffectsXml is responsible for initialising/restoring Quick Effects, though it only cares about existing decks when it's called (once) via CoreServices::initialize -> EffectsManager::setup. The deck count is still the initial 2 then, the default count PlayerManager is initialized with.
If any decks are added later on, e.g. when the default (4-deck) skin LateNight is loaded, effects.xml is not read again and the Quick Effect slots remain empty.
On next start it uses the deck_count attribute from soundconfig.xml.

IMO only EffectsManager should be responsible for effect defaults, not the Preferences.
Simply reading effects.xml again when a deck is added doesn't seem to work right away.
I'll check if it works with a slight refactoring of EffectsManager.

[ronso0]

I managed to fix it on top of #11695, though since that PR doesn't get attention I'll try to separate the commit.