Final fix for #12027

The issue is that track load triggers a cascade of calls in KeyControl which lead to unintended und annoying key offsets on track load with

• keylock ON, rate off center
• keylock mode "Keep current key"
• Reset Pitch on track load option enabled

Reproduce issue / confirm fix:

0. rate centered, pitch = 0, pitch_adjust = 0, keylock disabled
1. load a track
2. move rate slider off center -> key is off
3. load a new track
4. enable keylock
5. center rate slider
6. reset key with reset_key control -> key is reset, pitch = 0
7. load another track
   -> previously: pitch != 0
   -> now: pitch = 0

The issue stems from the fact that in #1222 we decided to not make the Key knob (pitch_adjust) jump when locking so that users can easily turn back to original key with it.
The consequence is that pitch_adjust should stay steady on track load.
The culprit is that BaseTrackPlayerImpl::slotTrackLoaded() did always reset pitch_adjust to 0 on track load.

The fix is to reset pitch (not pitch_adjust) to 0 in this case (key locked with "Keep current key").

Works beautifully and key tests pass 🤷‍♂️

FWIW actually we should refactor KeyControl as it seems too complicated with all its interleaved, potentially infinitly looping valueChanged() connections, as well as the interaction with BaseTrackPlayer and EngineBuffer, but I'm certainly not gonna refactor that, so consider this a quick but solid fix.