Switch to symbolic icons

This should improve the overall appearance in dark themes.
2 jobs for !2 with new_icon in 57 seconds (queued for 1 second)
detached
Status Name Job ID Coverage
  Build
passed build #576

00:00:32

 
  Distcheck
failed distcheck #577

00:00:25

 
Name Stage Failure
failed
distcheck Distcheck
checking that generated files are newer than configure... done
configure: creating ./config.status
config.status: creating Makefile
config.status: creating data/Makefile
config.status: error: cannot find input file: `data/icons/Makefile.in'
make: *** [Makefile:696: distcheck] Error 1
Running after_script
Uploading artifacts for failed job
ERROR: Job failed: exit code 1