Add visible, keyframeable track volume filters #1907

Create a Track Volume Audio Gain filter on demand when a track gain changes. Expose it in Filters with a track-specific heading, synchronize its name after track renames and moves, and keep timeline/header/filter-panel gain controls in sync, including animated values during playback.

Keep the filter last among visible track filters and prevent reordering it. It remains deletable; deleting it resets the track gain to 0 dB. Copying it creates a normal Audio Gain filter without the Track Volume marker.

I'm posting this for consideration and discussion. The current track volume sliders create a hidden gain filter that can not be manipulated by the user and does not support keyframes. This changes it so the filter is visible and the user can add keyframes. The track volume sliders move back and forth as the keyframes animate the volume.