Add pop-out player for audio/video in web UI (#14870)

Fix #11160

app/javascript/mastodon/reducers/picture_in_picture.js

@@ -0,0 +1,22 @@
+import { PICTURE_IN_PICTURE_DEPLOY, PICTURE_IN_PICTURE_REMOVE } from 'mastodon/actions/picture_in_picture';
+
+const initialState = {
+  statusId: null,
+  accountId: null,
+  type: null,
+  src: null,
+  muted: false,
+  volume: 0,
+  currentTime: 0,
+};
+
+export default function pictureInPicture(state = initialState, action) {
+  switch(action.type) {
+  case PICTURE_IN_PICTURE_DEPLOY:
+    return { statusId: action.statusId, accountId: action.accountId, type: action.playerType, ...action.props };
+  case PICTURE_IN_PICTURE_REMOVE:
+    return { ...initialState };
+  default:
+    return state;
+  }
+};