* input font name directly instead of a font picker because localized font family name is not supported by Avalonia
* fallback monospace font to default font
* remove unused code
* start background task only it is needed
* solve the problem that we can not use `ViewModels.Preference.Instance` until resource ready
* remove avatar sever settings
* 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.
