Add an option to copy before upgrading Realm files

[kraenhansen]

After https://github.com/realm/realm-studio/pull/1236 users are prompted when opening a Realm file, before the Realm file gets upgraded to the latest format. The prompt asks if the user wants to create backup or upgrade in-place, but I think we're lacking an option to help users that work on Realm files with older file formats.

Because some users use Realm Studio to open Realm files directly off a device or plan to bundle the Realm file into an app (both of which might not be using the latest file format), it might not be desirable to upgrade Realm files directly. We could add an option to copy the file (perhaps to a temporary location), upgrade it and possibly delete it when the browser window closes - a sort of "temporary" upgrade - or alternatively suffixing "-upgraded" to the file name. This would enable users to open older Realm files, without upgrading them: Effectively a kind of "read-only mode", since we're making a copy to a separate location before opening it and performing the upgrade.