Add indicator of unread content to window title when web UI is out of focus (#11560)

Fix #1288

app/javascript/mastodon/actions/app.js

@@ -0,0 +1,10 @@
+export const APP_FOCUS   = 'APP_FOCUS';
+export const APP_UNFOCUS = 'APP_UNFOCUS';
+
+export const focusApp = () => ({
+  type: APP_FOCUS,
+});
+
+export const unfocusApp = () => ({
+  type: APP_UNFOCUS,
+});