Sync data from another time tracker

Hello,

I'd like to synchronize data from another time tracker to ActivityWatch. Basic idea is to give users visual presentation (in AW) of AW data, such as active window, afk-state, etc. for every imported tracking session. This visual presentation could help them fill description of their activities in the time tracker with relevant data in case they forgot what they were working on. I'd like to synchronize data to AW both real-time and retrospectively.

What approach would you take if you would create such tool? Watcher for real-time syncing and importer for retrospective syncing?

- [x] I have searched the [issues](https://github.com/ActivityWatch/activitywatch/issues) of this repo and believe that this is not a duplicate.
- [x] I have searched the [documentation](https://docs.activitywatch.net/en/latest/) and believe that my question is not covered.

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