Word-wrap KMessageWidget text

Summary:
Word-wrap KMessageWidget text to prevent the window from being widened
when the text is very long.

CCBUG: 404232

Only `CCBUG: ` because word wrap only works where QLabel detects a word boundary;
it will not wrap a string with no spaces or other word boundaries (e.g.
"aaaaaaaaaaaaaaaaaaaa...")

Test Plan:
- Create a new text file with a 300-character name that has spaces in it
- In the KMessageWidget's error message, the file name is wrapped at the word
boundaries

Reviewers: #dolphin, #frameworks, cfeck, elvisangelaccio

Reviewed By: #dolphin, cfeck, elvisangelaccio

Subscribers: elvisangelaccio, cfeck, kfm-devel

Tags: #dolphin

Differential Revision: https://phabricator.kde.org/D18968