LayoutItem: better documentation for SetExplicitMinSize.

This is a bit counter-intuitive, at least to me, as forcing the minimal
size can lead to the item being smaller than it would otherwise.
Revert "FileTypes: make space for BBox titles"

This reverts commit f0eb9fcd9323903b2c1b7046a0a78e078663c6dc.

Fixing this in BBox is better.
BBox: fix minimal size when using layouted mode.

It is possible to set a custom layout on a BBox, which is then applied
to the children excluding the label. However, the label size must still
be used when computing the minimal size of the BBox in that case.