Parses the given mqpar.xml file (or, if not found, tries the 'txt_folder' + '/../../' folder (i.e. where the raw data should be)) to extract the full filepaths for all Raw files
getMetaFilenames(mqpar_file, txt_folder)
May return 'NULL' if no mqpar.xml could be found. Otherwise: data.frame with columns:
'file' (no path), 'path' (full path incl. names)
'file_no_suffix' (as 'file' but without suffix)
'CV' (CV term for filetype, e.g. for Thermo Raw)
Location of the mqpar.xml (can be empty, if unknown)
Fallback option: path to the txt folder (which contains evidence.txt, etc)