After some thoughs I choose to change the way the helper is shown in the KanaEditor.
The tab-based widget was not easy to use in real usage conditions.
So I changed the thing and made a new helper based on a single QListWidget.
Here is the new result on Linux/KDE :
And on windows XP :
I think the Japanese font installed on the windows I use are just crappy.
Else, I can't explain the difference in quality between the windows version and the Linux one.
Next time, I will try to compile it on apple's Mac OS X.
Last but not least, I made a Qt Designer plugin of this KanaEditor widget. And thanks to the guy of irc://irc.freenode.net#qt I fixed a damn semi-colon at the end of the class definition which was "preventing" my code to compile...
See you later.