Get icon spacing from stylesheet

[danirabbit]

Right now the icon spacing in the panel between the mic and volume icons is hardcoded. For future proofing, we should get this value from the padding of .composited-indicator

[danirabbit]

#318 uses css instead but we should still probably do something like using border-spacing in GTK4 maybe in the panel CSS itself? So I'll leave this open as a reminder