Modify Metadata display to differentiate constant / file-dependent / measurement-dependent

[jwuttke]

Now that we have a generic metadata handling, we can implement the following improvements:

• In the "Metadata" display, use different colors to differentiate constant from variable metadata:

  • black, if the metadatum is the same for all files;
  • dark violet, if the metadatum differs between files, but is the same for all measurements in one file;
  • dark blue, if the metadatum varies from file to file.

• Modify the response to checking a metadatum:

  • in the black case, disable the checking;
  • in the violet case, show values as a column in the "Files" display;
  • in the blue case, continue show values as a column in the "Measurements" display.