[image-wallpaper] Port to Kirigami.FormLayout and use twinFormLayouts

Summary:
Porting to Kirigami.FormLayout and using twinFormLayouts in order to ensure alignment with the main layout.

NOTE: Vertical spacing is still clearly wrong, but I could fix that as well if someone could offer a suggestion on how to do it.

Test Plan:
D19932 needed before testing.

Before:
{F6707737}

After:
{F6707739}

Before:
{F6707742}

After:
{F6707740}

Everything still worked.

Reviewers: #plasma, ngraham, mart, hein

Reviewed By: #plasma, ngraham, mart, hein

Subscribers: hein, davidre, mart, abetts, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D19873