Fix the positioning of the `add` button when on Gtk style,
in my XFCE install (and thus, may differ a bit from the
Gnome3 one ) the Plus button to add cylinders and weigths
got really strange placements, this seems to fix this,
I also tested on Oxygen and nothing broke there.
Signed-off-by: Tomaz Canabrava <tcanabrava@kde.org>