Allow gradual fading for potmeter controls, e.g. the crossfader

[jus]

This is a follow up to the discussion in lp:197696 "Crossfader cannot do softstep"

We should add support for gradual fades to potmeter controls and also _up_small and _down_small controls.

This would allow e.g. to smooth, gradual crossfade when the control is held down. The current behaviour is cumbersome where we have to click 50 times to move the crossfader from the center to the outer position in small steps.

[RJ Skerry-Ryan (rryan)]

Keyboard-wise we don't allow repeats so holding down a key won't smoothly fade. To allow a smooth-fade without jamming the key over and over we'd have to enable repeats. The downside of that is if you hold down 'd' for too long then it would be the same as repeatedly pressing d over and over which would cause deck 1 to rapidly start and stop. Maybe we should try and inspect the control the key is mapped to and if it is a potmeter then allow repeats?

[Daniel Schürmann (daschuer)]

In the advanced auto dj branch lp:~smstewart91/mixxx/advanced_autodj we have a the option to use auto DJ fades outside Auto DJ. We have two new "fade now" controls which simply starts the fade in a specific direction. The speed is taken from auto DJ setting.

For me this is a suitable solution to solve this bug.

[Swiftb0y (swiftb0y)]

Mixxx now uses GitHub for bug tracking. This bug has been migrated to:
https://github.com/mixxxdj/mixxx/issues/6800