Updated sources to more modern C# - nullable reference types support etc
Started to update projects starting from 'z' and 'u' utilities
Merged changes from the branch 'dev_v22'
Run tests - pass
Added one more argument validation to FileDialogCommand ctor
Removed an unimplemented by MS property RestoreDirectory from FileDialogCommand
Added more properties to FileDialogCommand with handlers in OpenFileCommand and SaveFileCommand that allow better setting up standard dialogs
Added missing XML comments to these three new commands