Small proposal to manage the CPU high load issue: Schedule synchronization at a fixed time (daily)

### Describe the feature you'd like to request

Hello

**Thank you very much for sharing your great and very useful work!**

### My request addresses this issue

When using WebDAV, floccus takes nearly 5 minutes to synchronize an xbel file that is 1.7 MB in size to:
1. Lock the xbel file
2. Download and deserialize the xbel file
3. Compare the bookmarks and synchronize them with the data in RAM
4. Write the xbel file back and remove the lock

That’s why it would be great if we could schedule synchronization for a fixed time, e.g., always during the lunch break. Then the CPU load wouldn’t be a problem for the user 😍

### Describe the solution you'd like

### My simple proposal 🙂

**Could you therefore please:**
1. Offer the option to schedule synchronization for a fixed time every day (keep it simple!)
2. If the xbel file is locked, have the sync wait, e.g., 10 minutes
3. If the computer isn’t running at that time, ideally, have the synchronization catch up as soon as the coputer is restarted

Thanks a lot, kind regards,
Thomas

### Describe alternatives you've considered

* Currently, users can set the sync to run every 24 hours, for example.
* But then it still happens that the CPU load is very high while the user is working.