Don't elide strings in the HeaderBar when there's plenty of space

Summary:
The parent `ColumnLayout` had `Layout.fillWidth: true`, but its children didn't, and
they don't inherit it automatically.

This patch sets that property for the children so that strings in the HeaderBar don't
get elided.

BUG: 406483

Test Plan: {F6774565, size=full}

Reviewers: mgallien, #elisa

Reviewed By: mgallien, #elisa

Differential Revision: https://phabricator.kde.org/D20578