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