messagelist: fix horizontal scrollbar when applying column sizes

Summary:
On startup the column sizes were calculated for a viewport without
vertical scrollbar, and then not adjusted when the vertical scrollbar
later appears. My fix is to simply always recalculate column sizes
when the viewport is resized at any point. If I resize the mainwindow
I still don't want a horizontal scrollbar under the messagelist.

This removes the need for showEvent since any change of size triggers
resizeEvent anyway. I also can't see any use for recalculating the
sizes on model reset (e.g. switching folders), QHeaderView appears to
just keep the column sizes when doing that (might be my own QHeaderView
fix, actually).

Test Plan: starting kmail, switching folders, resizing columns...

Reviewers: mlaurent

Reviewed By: mlaurent

Subscribers: dvratil, mkoller, #kde_pim

Tags: #kde_pim

Differential Revision: https://phabricator.kde.org/D5724