Continuation of https://git.key-project.org/key/key/-/merge_requests/627

This branch adds a new dropdown list and button to the toolbar, used to undo the last action performed by the user.

Why?

After e.g. unsuccessfully applying a strategy, a lot of proof steps may have been added. Removing these was an annoying manual operation previously. Now, the user can just undo the last action.

Features and Impact

See screenshot above. Currently supported user actions:

• loading a proof
• manually applying a rule
• executing a strategy
• closing (several) goals by SMT
• running auto mode (either via toolbar button, or proof tree popup "Apply strategy", or sequent popup "Apply rules automatically here")

Open Questions

It is not clear how to limit the size of the popup menu.

Review

Should be fairly straightforward.