Dive list: make saving / restoring column widths actually work

It's a testament to how much I mess around with things that I hadn't
noticed that saving the column width doesn't actually work. Or actually,
saving them worked, loading them back failed as it was done too early and
the setColumnWidth() calls had no effect - and so the next time we quit
subsurface, the default width of 100 was written over all the saved
values.

This seems like an incredible hack but it has the advantage of actually
working. I look forward to someone with better insides into the inner
workings of Qt to properly fix this.

Signed-off-by: Dirk Hohndel <dirk@hohndel.org>