With these changes, a multi-line message cannow be copied-into, viewed and copied-out-of a Stash:
* Simplify parsing in QueryStashes, by passing the `-z` argument to `git stash list` for item separation.
* Query full Message (`%B`) from stash (list-item still shows only first (Subject) line, tooltip shows full Message).
* Add "Copy Message" command in stash-item context-menu.
* Make the "Message:" text-box in StashChanges dialog support multiple lines, without changing the layout.
* remove dotnet-tool.json because the project does not rely on any dotnet tools.
* remove Directory.Build.props because the solution has only one project.
* move src/SourceGit to src. It's not needed to put all sources into a subfolder of src since there's only one project.
