add hotkey to edit track comment in deck, take 2

[ronso0]

This is a retake of #15521 which has accidentally been merged prematurely

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Here's yet another small mod I find very helpful for preparing/editing tracks while using only the keyboard (+ controller).
If you use a skin that has a comment widget in decks (WTrackProperty), press Alt+[deck number] to open the property editor.

So the workflow is simply this, no mouse/trackpad involved:

• Alt+1 for deck 1
• edit text
• Enter, done

Plus these mods:

• show only first comment line in editor
  allows to add tag-like comments to the first line while keeping stuff like lyrics or release info out of sight
  (in the library only the first line is displayed anyway)
• add linebreak with \n
  allows to push above mentioned extra info down

The hotkey currently works via QAction in each WTrackProperty dsiplaying the comment, as that was the quickest way for me to get this feature.
It is failproof in the sense that there should be no issue with even multiple comment labels visible for one deck.
I'd like to share it, though I am not 100% sure it can/should be integrated as is, simply because the hotkey can not be customized (didn't check if it would overwrite keys from *.kbd.cfg or would be overwritten by such).

The alternative to QAction would be a ControlProxy like we have for the track menu control. It's more elaborate, but allows to customize the hotkey via Custom.kbd.cfg