Widget for putting Icons like in Gimp

[Olivier GUERINEAU <oguerineau altair com>]

Hello

in the main window of Gimp,  there is a widget containing icons, with an
interesting behaviour.
When the size of the window changes, the icons are reorganized :
for example if the window is very large, then the icons are on one line,
and if the window is narrower then icons are displayed on more than one
line.
Does anyone know which widget is used and how, for enabling this
behaviour ?

In advance thanks
Olivier