[Notifications] Only let details label grow, never shrink

Authored by broulik on Feb 13 2020, 9:27 AM.

Description

[Notifications] Only let details label grow, never shrink

Avoids repeatedly resizing the dialog when copying many files.
It will still resize when the speed label comes and goes but at least when copying many tiny files it won't be as obnoxious.

BUG: 417489
FIXED-IN: 5.18.1

Differential Revision: https://phabricator.kde.org/D27344