Add a nice GUI for downgrading in the updater

We have enough information in the `db.xml.gz` files to downgrade relatively reliably (with multiple update sites, we must trust the servers' time to be synchronized well enough).

To downgrade, we should not ask the user for a date, but rather have an overview of the files that changed, in a chronological order.

As a bonus, this view would give us also a very nice graphical representation of Fiji's development from the users' view.

Start by examining the updater and the information available in the db.xml.gz files, especially how multiple update sites and server times affect downgrade decisions. Done means providing a GUI that shows changed files in chronological order and lets users choose a downgrade without entering a date, with the development-history visualization as an optional enhancement.

Written by the indexing model from the issue text.