[Win32] DDE and registry handling improvements

- Win32Slurp class had become more Unicode-friendlier and should now allow to have Abiword installed in folder with arbitrary name. (untested)
- Abiword should no longer register its filetypes if running from removable drive. (debatable)
- DDE commands now operate in Unicode mode and allow to open files with arbitrary names in existing Abiword instance.

- Fixed small bug in ut_go_file.cpp relating to detection 'plain' filenames in Windows.