Remove the s character at the end of the extension of suggested .labels filenames

JSON files for behavior tags are generated with extension .labels. In line with Choreography files that exhibit the .spine and .outline extensions with no ending s, .labels could be replaced by .label whenever a file name is suggested, per default.

The .labels extension will still be supported anyway, along with .label.

This issue also aims at setting an official extension in stone, as file listing functionalities would be greatly sped up if they could disregard some files just looking at the extension. Choreography files will be identified as bearing the .spine or .outline extension, FIMTrack v2 csv files the .csv extension, trx.mat files the .mat extension and, as per this issue, json files for labels will exhibit the .label extension. Note that the full file name will not be trusted for table.csv or trx.mat files, as variations in the stem already exist for these file types.

EDIT: .nyxlabel should also be admitted as an extension.