Make last loaded snapshot available in "ReaLearn: Take mapping snapshot"

[helgoboss]

• That way we could make it easier for the user to update the last-loaded snapshot
• Introducing a snapshot selector might be a good idea

[helgoboss]

The last-loaded snapshot is saved per tag! If the "Take snapshot" target is configured to work on multiple tags, the snapshot taking is only executed if the last-loaded snapshot of all the given tags is the same! Otherwise it will do nothing, it's safer. It's best to avoid working with multiple tags when using the "Take snapshot" target, it quickly gets confusing. A single tag is enough.