Ok, now, I have the ibus systray icon that work fine and the toolbar is displayed correctly.
(since I use only one method to write japanese it was not useful for me, but it show that the package is well done.)
Now, I can write japanese directly in GTK apps whereas before I couldn't.

But, even if the variables for the default input method are set :
gyan011@gyanopolis ~ $ env | grep ibus
QT_IM_MODULE=ibus
XMODIFIERS=@im=ibus
GTK_IM_MODULE=ibus
I can use ibus only when I launch application from a konsole.
The only workaround I found is add the
export GTK_IM_MODULE=ibus
export QT_IM_MODULE=ibus
export XMODIFIERS="@im=ibus"
in /usr/bin/startkde but that's a dirty solution and I don't know where to set these variables. It's the last problem I see with this very useful new package.
@atolboo.
Since the variables are automatically set when ibus is started, it's useless to write
ibus-daemon --xim to use ibus in a konsole.