icons: temporarily use more Default icons for Legacy theme
Followup to c73c2744.
All found missing icons in the Legacy theme now use the corresponding icon from Default icon theme as a stop-gap measure to not have missing icons in GIMP 3.0.
I think I at least got most of the icons, but it's hard to know, the GUI of GIMP is vast. The icons in this MR were previously shown as a white square, or in some cases the Wilber icon for some reason.