OOUI PHP: Use 'OOUI' namespace instead of 'OoUi' class name prefix
- Added 'namespace OOUI;' and removed 'OoUi' class prefixes from all PHP source files with simple search-and-replace.
- Renamed the files accordingly.
- Updated Doxyfile, I hope this works like I think it does.
- Updated widgets demo and comparison test runner.