[WidgetExplorer] Fix blurry previews

Summary: Round the cell width to avoid getting blurry previews.

Test Plan:
Before:

After:

Reviewers: Plasma, VDG, ngraham, davidedmundson

Reviewed By: Plasma, VDG, ngraham, davidedmundson

Subscribers: plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D21030