- use the overall volume managed by the playlist rather than
     aout_volumeGet() that now only works when a aout is running.
     This is no problem since the skins2 volume slider functionally
     reflects this overall volume managed by the playlist anyway.
   - stick to what include/vlc_aout_intf.h lets us know about volume
     boundaries(as of today, AOUT_VOLUME_DEFAULT and AOUT_VOLUME_MAX)
     and infer volume to be in the [0., AOUT_VOLUME_MAX/AOUT_VOLUME_DEFAULT]
     range as requested by some functions taking a float as parameter.

This is generic and reusable on non-POSIX platforms.

A more proper fix was made in 3567c033.

pb_peak and pf_peak have only two elements.
(regression from dcf3f71b)

Also fixes a mistake in 229c807c
where isCurrent == isSelected

This fixes #7165 and can be backported.

Several of UI & control plugins follow the volume and mute status
through the playlist instead of the aout. This patch works around
that limitation.

int64_t is not supported natively by the Qt object system.

These helper handle mute (i.e. zero gain) and keep track of the volume
across sessions on a per-output basis. This is intended for the "old"
audio outputs that cannot amplify natively.

This is more flexible than the current approach:
- Gain can be changed if the aout has a custom (native) volume control.
- The callback approach does not break the encapsulation principles
  (so audio_output_t/module_need() could be used somewhere else).

Amplifying dummy is useless. Also amplifying S/PDIF is not possible.