Make a great first-user experience

To avoid situations like this for aw-qt, where people expect a window to open and don't notice the tray icon: https://forum.activitywatch.net/t/activity-watch-wont-launch-on-windows/1389

Possible features, in increasing order of difficulty:

- [x] When app is started and already running, open webview.
- [x] Add notification when ActivityWatch starts (as we used to), add setting to turn it off.
- [ ] Proper onboarding experience when app is started for the first time (or onboarding never shown), introducing ActivityWatch and the tray icon.