[sddm-kcm] Adjust Background label and button

Summary:
This patch adjusts the "Background:" label (adds padding and changes its alignment) and button to align with the changes implemented in D19077. It also moves the background preview button to below its corresponding label.

(P.S. The padding above the Background: label is controlled by patch D19077 for the most part.)

Test Plan:
Before (with patch D19077 applied):

After (with patch D19077 applied):

For reference,
before (without patch D19077):

after (without patch D19077):

Reviewers: VDG, Plasma, filipf, ngraham

Reviewed By: VDG, filipf

Subscribers: GB_2, abetts, plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D19209