[wallpapers/image] Put spinbox labels inside the spinboxes

Summary: They look a little nicer this way, and can be localized more easily.

Test Plan:
Before: {F7871094, size=full}
After:{F7871093, size=full}

Reviewers: #vdg, #plasma, mart

Reviewed By: #vdg, #plasma, mart

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D26529