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:

Reviewers: mgallien, Elisa

Reviewed By: mgallien, Elisa

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