macosx: Add stereo_pan filter

Add stereo pan filter to Advanced Audio Effects panel.

Add enable button for each filter, as pan audio effect has effect over
the whole spectrum (always noticeable), thus, if always enabled, it
makes pitch adjustment unusable.

Additionally, for pitch adjustment:
- Update the step of the scale to 0.25
- Fix the default value to 0.0 (was previously 0.25)

Closes issue #25872