Allow setting an interval to refresh database when saved in cloud

**The problem**
It often takes up to fifteen seconds to open the password database from Nextcloud. AFAIK, this is due to KeePassDX requesting the newest version of the database every single time I want to, say, autofill a password (even though there is no newer version available).

**Describe the solution you'd like**
It might make more sense to automatically update the database every x hours. This wouldn't be the default behavior of the app, but an advanced setting for those saving their databases in a cloud storage, accessed by KeePassDX via the Storage Access Framework.

When a user enables this feature for a specific database, the database is no longer downloaded on unlock – instead, the database merely has to be unlocked, significantly reducing the time a user has to wait.

**Describe alternatives you've considered**
Perhaps KeePassDX could somehow check if there is a newer version available upon unlock, instead of defaulting to always downloading the database. This wouldn't be as smooth as the above though, as there would still be cases where the database has to be downloaded before unlocking. Ideally, you would combine the two approaches: keep the database up to date in the background, but only when there actually is a new version.

Thank you for working on a free software project!

No files or tests are named. Start by tracing database unlock and download behavior through the Storage Access Framework, then determine how per-database refresh intervals and background updates fit the existing flow. Done means an optional advanced setting can avoid downloading on every unlock while keeping the cloud database refreshed.

Written by the indexing model from the issue text.