Get Info has a Volume slider (−100%…+100%): iTunes' own per-song volume
adjustment, which the library already imported but never used. It scales
the volume slider while that song plays; a boost can't pass full volume.
Works on one song or several (Mixed when they differ), and undoes.
Get Info's Previous/Next buttons are gone: since the combined multi-song
editor they were always disabled (and blank in some themes).
The laser settings draw the rig — speaker and mirror, laser, beam, and the
screen at its distance and angles — as a small textbook line drawing that
labels distance, angles and dot size in place of the sliders' numbers.
Preferences section headings are bold and 2 px larger than the text.
