Preview effect of configuration changes (delete/add)

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### Feature description
During the setup process of the desktop app, and when changing the configured sync folders, users should per default be shown a preview of the effect of their changes (a dry run) in a visual, easy to grasp overview (file tree, red for deleted folders, green for keep with +/- for clarity, unfolded along the path until first affected level).

Background: Preventing to accidentally delete many files for new users I have personally and my non-tech friends been severely burnt by setting up the desktop app, only to find it either deleted all files in my local or my remote folder. This is absolutely terrible UX and can lead to significant data loss... this is supposed to be a synchronization and backup solution, the worst it can do is to nuke the folder it is supposed to duplicate! Please reconsider this design :)

This goes along with
https://github.com/nextcloud/desktop/issues/6625
https://github.com/nextcloud/desktop/issues/6162
https://github.com/nextcloud/desktop/issues/5402
https://github.com/nextcloud/desktop/issues/5034
https://github.com/nextcloud/desktop/issues/4231