Python 3 porting

[Kai Willadsen <kai willadsen gmail com>]

Hi all,

The Python3 branch now contains a quite-incomplete port of Meld to
Python 3. It should Mostly Work (tm), but chances are excellent that
using it for more than five minutes will find problems.

The major issue stopping this being merged to master (other than lots
of testing) is that filename handling is known broken. Our unicode
filename handling wasn't previously the best, but we tried to stay
aware of bytewise handling of filenames and being careful with
maintaining a separation between the on-disk filename and the display
name.

Python 3 now strongly encourages people to believe that filename are
unicode and GTK+/gio's interaction with Python basically enforces this
to be the case [1]. Chances that this new assumption has broken
unicode file handling in some places is very, very high.

Bug reports for specific breakage are welcome, but please just don't
expect everything to work just yet.

cheers,
Kai

[1] ...making it impossible to correctly handle files with bad
filesystem encodings, among other issues. Woo.