Added placeholders across the application

changes/changelog.d/750.enhancement

front/src/components/audio/album/Widget.vue

front/src/components/audio/track/Widget.vue

front/src/components/favorites/List.vue

front/src/components/library/Albums.vue

front/src/components/library/Artists.vue

front/src/components/library/Radios.vue

front/src/components/manage/moderation/DomainsTable.vue

front/src/components/playlists/PlaceholderWidget.vue

front/src/components/playlists/PlaylistModal.vue

front/src/components/playlists/Widget.vue

front/src/views/content/libraries/FilesTable.vue

front/src/views/playlists/Detail.vue

front/src/views/playlists/List.vue

front/src/views/radios/Detail.vue