dataio: handle correctly non-ascii filenames

The only way in Python to support every filnames is to use unicode strings.
HDF5 was doing fine fine, but TIFF needs to explicitly go back to the current filesystem encoding.

Now expects the filename to always be unicode, to force the correct handling.
Update the extensions to be unicode.

Add also add test cases.
gui: handle better non-ascii filenames

Filenames should be always encoded in unicode to work correctly.

So force all the filename-related functions (that I could find) to use unicode.

That fixes a bug when the user would try to save a file in a directory with non-ascii characters.

Also detect if the currect directory doesn't exists (anymore), and fallback to the default directory.

This fixes another bug which could happen if the directory was moved/renamed.