Implement group sync

[evur]

Some small modifications to sync groups.

This is a really important feature, which is long overdue and the expected behavior for many users.

Fixes #3045

ToDo

• Add a setting to disable the feature
• Add unit tests
• Sync root group icon if possible (maybe this needs to be fixed in the merge function idk)

NOTE: I personally won't implement these todos, as I'm not experienced with the codebase and Qt. Feel free to implement them and add them to the PR.

Testing strategy

Create a KeeShare between two databases, add groups and entries to it and set icons and custom icons.

Type of change

• ✅ New feature (change that adds functionality)

[michaelk83]

AFAIK KeeShare is being reworked for 2.8.0, so you may want to wait with further work on this.

[evur]

AFAIK KeeShare is being reworked for 2.8.0, so you may want to wait with further work on this.

I just created this PR so it's possible for maintainers to continue working on it and for others to easily get this functionality. This means that I won't work on this PR as I don't have experience with KeePassXC's codebase and don't know what exactly is getting reworked.

[droidmonkey]

I might merge this work with what I already have done for KeeShare refactor. Thank you for the contribution!

[BenKluwe]

I'd love to add to this but I can't figure out how to do it. In the code I've added a setting on the group's keeshare settings page for recursion but since I'm operating on a fork and the pr is from a different repo github isn't allowing me to add my commit -- and rightly so --