Add "Balance" option for per-channel (e.g. left, right) volume sliders

When checking the "Balance" button, a volume slider for each channel (e.g. left, right, center, subwoofer, etc) is shown.

When opening settings with different volumes in some of the channels, "Balance" is checked automatically.

Furthermore, when unchecking "Balance" again, all channels are set back to a uniform volume. Normally, plasma-pa adjusts volume by keeping the difference between channels in tact, so means of explicitly assigning all channels in one go is added.

Also, QQC2 Slider has an explicit onMoved signal which is only fired when the user explicitly moves the slider. This should remove the need for the convoluted "ignore changes" property dance that was present before.

Technically, we could also do per-channel volume for streams, which isn't done in this patch, as I am not sure how useful being able to play music on the left speaker and watching a video on the right is.

Latest iteration: