column: Fix the natural width

Set the natural width of the column from the child's one to what is
needed for the child to reach the maximum allocatable width.

This not only makes more sense but it will allow HdyColumn to serve as a
container setting a natural size to its child by giving the same value
to the maximum width and the linear growth width.

Librem5/libhandy#87
5 jobs for column-natural in 7 minutes and 58 seconds
latest
Status Job ID Name Coverage
  Build
passed #9762
librem5
build-debian-clang

00:02:34

passed #9760
librem5
build-debian-gcc

00:02:46

38.4%
passed #9761
librem5
build-debian-gcc-static

00:01:59

passed #9764
librem5
build-debian-package

00:01:47

passed #9763
librem5
build-fedora-gcc

00:07:34