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