fix inversed singular and plural in translated strings when translation libraries are not found
various fixes and improvements to run system commands

- add RunShellCmd to run a command by the shell
- add RunSysCmdOnSelected and RunShellCmdOnSelected to run commands on
  selected songs, either one command per song, or if $songs is used one
  command for all songs
- fix incorrect filenames in custom commands of the export plugin and
  nowplaying plugin when non-utf8 filenames are used