folderview: use -symbolic version for CompactRepresentation icon by default
When not using a custom icon, we want to make sure that we use the -symbolic version (if it exists) of whatever icon is automatically chosen for the CompactRepresentation's icon, so the Panel icons will always be monochrome when using a mixed mono/colorful icon theme.
BUG: 416041 FIXED-IN: 6.0
To test, change the thickness of the panel between 38px and 40px and see how the automatically-chosen icon is monochrome and remains so at both sizes.