Fixes: broken calculation of toolbox layout

* last button with multiple sections on row was cut off at end (spacing ignored)
* separators could appear on the top side of the toolbox
* also makes 294601 disappear
(unsure why, could be due to too wide section widgets, let's just assume its gone for now)

REVIEW: 104432
BUG: 294601
FIXED-IN: 2.5 Alpha 2