align checkable widgets in menu items

Summary:
Before

{F5714193, layout=center, size=full}

After

{F5714196, layout=center, size=full}

Test Plan: * open a context menu with icons and check boxes

Reviewers: #breeze, #vdg, hpereiradacosta

Reviewed By: hpereiradacosta

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D10480