Alternative for: "If possible save the last selected node in the proof, to reselect it upon loading."

[wadoon]

Alternative version for #3324 using the settings instead of storing it in the proof.

Using a path encoding of the current node (list of child indexes) and a run-length encoding to keep it small.

Missing: Finding the right spot where the KeYUserProblemFile is available and the proof is loaded. The current place is unsuitable.

[wadoon]

@unp1 I now call getMediator().nonGoalChosen(node) with a valid node, but it is not selected. Where is the other place with the logic that the last node should be taken?

[codecov[bot]]

Codecov Report

Attention: 42 lines in your changes are missing coverage. Please review.

Comparison is base (603b2ca) 37.93% compared to head (66b0aa4) 37.93%.

Files	Patch %	Lines
.../uka/ilkd/key/proof/io/OutputStreamProofSaver.java	25.00%	14 Missing and 1 partial :warning:
...core/src/main/java/de/uka/ilkd/key/proof/Node.java	0.00%	12 Missing :warning:
...n/java/de/uka/ilkd/key/settings/Configuration.java	9.09%	10 Missing :warning:
...uka/ilkd/key/macros/scripts/ProofScriptEngine.java	0.00%	2 Missing :warning:
.../src/main/java/de/uka/ilkd/key/nparser/KeyAst.java	90.90%	1 Missing :warning:
...de/uka/ilkd/key/proof/init/KeYUserProblemFile.java	75.00%	0 Missing and 1 partial :warning:
...n/java/de/uka/ilkd/key/settings/ProofSettings.java	90.00%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #3380 +/- ## ========================================== Coverage 37.93% 37.93% - Complexity 17030 17038 +8 ========================================== Files 2060 2060 Lines 126300 126409 +109 Branches 21304 21324 +20 ========================================== + Hits 47913 47958 +45 - Misses 72499 72560 +61 - Partials 5888 5891 +3 ```

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