refactor: re-work duration and time calculations

src/Pages/AddOrEditPlaylist.vue

src/components/emissions/ModalCreate.vue

src/components/emissions/ModalEdit.vue

src/components/filemanager/Files.vue

src/components/shows/PlaylistSelector.vue

src/components/shows/Schedules.vue

src/components/shows/TimeSlotRow.vue

src/mixins/playlist.js

src/mixins/prettyDate.js

src/stores/playlists.ts

src/util.ts