[kcmkwin/desktop] Elide "Show animation when switching" checkbox text

Summary:
Otherwise the combobox containing the switching effects escapes from the page. Same fix
as D24885.

BUG: 403151
FIXED-IN: 5.17.2

Test Plan:
Checkbox text now elides when the window is made very narrow, and combobox no longer gets
cut off.

Reviewers: #kwin, zzag, davidedmundson

Reviewed By: #kwin, zzag, davidedmundson

Subscribers: kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D24988