GTK+ 2.6.4

[Matthias Clasen <mclasen redhat com>]

I'm doing a quick 2.6.4 release for some important fixes, and took the
occasion to fix the translation of the only Q_()-marked string in 
GTK+ 2.6 (approx. half the translations included the prefix in the
translated text). I do not blame the translators, since we did not
have a translator comment for this message in the 2.6 branch. I've
added one now.

Note that GTK+ HEAD uses Q_() in some more places (all with comments
though).

Thanks, Matthias