Fix layout of resolution selection

Summary:
When there is only one widget inside the layout, it would have double
margins. In case of the slider, the top and bottom margins are slightly
off (same as before), but at least on the left and right they now fit.

Reviewers: #plasma, #vdg, ngraham

Reviewed By: #vdg, ngraham

Subscribers: andreaska, abetts, ngraham, broulik, zzag, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D14326