Move subtitles flags to their own directory

gui/slick/images/subtitles/flags/afr.png

gui/slick/images/subtitles/flags/amh.png

gui/slick/images/subtitles/flags/ara.png

gui/slick/images/subtitles/flags/arg.png

gui/slick/images/subtitles/flags/asm.png

gui/slick/images/subtitles/flags/ave.png

gui/slick/images/subtitles/flags/aym.png

gui/slick/images/subtitles/flags/aze.png

gui/slick/images/subtitles/flags/bak.png

gui/slick/images/subtitles/flags/bam.png

gui/slick/images/subtitles/flags/bel.png

gui/slick/images/subtitles/flags/ben.png

gui/slick/images/subtitles/flags/bis.png

gui/slick/images/subtitles/flags/bod.png

gui/slick/images/subtitles/flags/bos.png

gui/slick/images/subtitles/flags/bre.png

gui/slick/images/subtitles/flags/bul.png

gui/slick/images/subtitles/flags/cat.png

gui/slick/images/subtitles/flags/ces.png

gui/slick/images/subtitles/flags/cha.png