Allow editing method source for merge conflicts

[codeZeilen]

For merge conflicts, it would be nice to be able to edit the source code of a conflicting method and create the merged version manually.

Current state is at branch Squot/swa-chooseAnother:.

Considerations:

• Do not forget that the same protocol also should work for non-package artifacts (see SquotConflict)
• The current solution based on SquotMergeDiffNode>>#editTo: might still turn out to be incomplete, if it turns out that we also have to change the incoming snapshot. If this happens, more fundamental discussions become necessary.
• SquotMergeDiffNode instances are UI objects that represent the state of the SquotMergeTool and not any underlying version control semantic
• Beware of modifications to code artifacts other than methods, scripts should probably work, class definitions might be over the top at this point

Remaining todos

• [ ] Think about selector for SquotMergeDiffNode>>#editTo:
• [ ] Add an editor to the SquotMergeTool
• [ ] Add the new timestamp when editing a method in the SquotMergeTool