Fix the header layouts for EntryDetails and Page components

Summary:
This ensures that the header is the correct width, and further that
the text does not disappear when there is no space for the text (so
that it elides as intended).

BUG:413433
BUG:413440

Test Plan:
Without patch: Resize a dialog to make the text not fit, and it disappears
With patch: Resize dialog, text is elided as one would expect, and the header row takes up the correct amount of space

Reviewers: ngraham, #knewstuff, #frameworks, #plasma

Reviewed By: ngraham

Subscribers: ahiemstra, kde-frameworks-devel

Tags: #frameworks

Differential Revision: https://phabricator.kde.org/D25067