[KCM] Make the warning message wider

Summary:
Making the message wider improves the presentation, especially for wordy languages
where the message may become very tall.

Test Plan:

Reviewers: VDG, Plasma, romangg, dhaumann, ndavis

Reviewed By: VDG, dhaumann, ndavis

Subscribers: dhaumann, plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D24371