Should close #7696

Ref #7929

This restores the brittle logic from VLC version 2.0.x (fixes #8499).
I strongly suspect there are some false positives left, which would
lead to ghost video outputs.

This tries to make the aout filters reusable outside of the aout
(refs #8447).

Since audio filters and visualization all have a priority of zero,
forced module loading has no effects.

Previously, noise occured in stereo mode because only
half of the buffer was zeroed out.

fixes #8507

macosx: remove http proxy settings from simplified preferences since the networking core will automatically use the system settings now

redefining notifyLayout is mandatory for sliders, because the default
notifyLayout based on position doesn't work well for this complicated control.
This function is called when visibility is turned on or off.

--audio-visual should be used instead. Using --audio-filter does not
play well with video output recycling.

Especially in the case that volume-save was set to false...
This adapts to behaviour of the other aout modules.

fixes #8501