wlt: toolkit: implement proper minimal sizes

wlt: toolkit: implement proper minimal sizes

A widget may have different constraints regarding minimal sizes and the
size it occupies. Therefore, we need to pass a hint with the minimal size
together with the prepare-resize round.

The catch-all user can then decide to resize the catch-all field until it
is big enough to hold all minimal-sizes.

Signed-off-by: David Herrmann <dh.herrmann@googlemail.com>