overview (hot) cue markers: add play_from_cue functionality

[mixxxbot]

Reported by: ronso0
Date: 2019-11-18T18:48:27Z
Status: Confirmed
Importance: Wishlist
Launchpad Issue: lp1853043
Tags: cue, hackathon, usability

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this would be a great addon to the new hotcue markers & menu funcions!
should behave exactly like hotcue_N_activate to play from cue until button release.

as far as I can tell, there wouldn't be any interference with the current hover functionality.
I just checked it this way:

• press a hotcue marker in the overview
   (optionally keep the mouse button pressed)
• press corresponding keyboard or controller button to play from cue
  = track plays from cue
  = time displayed at playpos marker increases