Use Python Unicode encoding for gettexted strings

Use Python Unicode encoding for gettexted strings

Python 2.5 strings encoding handling seems not always consistent
between various modules, so we must use Python Unicode strings and
thus reencode strings when writing to stderr/stdout.  This commit
applies changes proposed on lilypond-devel a while ago in thread
"Gettexted messages encoding with Python 2.5".

Even if GUB still uses Python 2.4, LilyPond packages on some distros
(e.g. Fedora) already use 2.5, so it's important to fix this.  I think
these changes are also compatible with 2.4, but anyway I must check
GUB binary when it's out.

By the way, further localization work is needed: localize musicxml2ly
strings, and fix some nitpicks.