Adjust column to its contents when it's made visible

When a hidden column is shown then Qt uses either the previously used
size (e.g. the size we set to the default size when we restore a hidden
column) or the default size. If a column has the default size after
being shown then it's very likely that the column wasn't resized to its
contents.

(cherry picked from commit 8ec94dc88ff1601a80c888fa41402704101feade)

• GnuPG-bug-id: T7227