Add a "snap to zero crossings" snapping option

Snapping currently has limited value to anything but music. Snapping to seconds of frames has niche uses at best.

We currently have a "Select -> At Zero Crossings", which is an action.

Having it as a toggle allows to transform an editing workflow as such:

> **select audio -> Z -> cut, select different audio -> Z -> cut**, ...
> **turn on snapping -> select audio -> cut, select different audio -> cut**