[WidgetExplorer] Fix blurry previews

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

Test Plan:
Before:
{F6809406}

After:
{F6810901}

Reviewers: #plasma, #vdg, ngraham, davidedmundson

Reviewed By: #plasma, #vdg, ngraham, davidedmundson

Subscribers: plasma-devel

Tags: #plasma

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