Refactor: Move app files to src/app subdirectory inside app namespace

- Add HAVE_CONFIG_H wrapper to #include "config.h"
- Removed widgets namespace (it's in app now)
- Move some functions from src/file/file.h to src/base/cfile.h
- Move Vector2d to base library
- Rename MenuItem2 to AppMenuItem
Remove dependency of "undo" library with "base"
Update src/README.md