Support Kirigami.twinFormLayouts in individual wallpaper plugins

Summary:
This patch does the groundwork for indvidual wallpaper plugins to be able to use Kirigami FormLayout with twinFormLayouts,
finally allowing us to easily ensure that both the main and child layout are aligned without hacks.

Test Plan: Test with D19873

Reviewers: mart, #plasma, ngraham

Reviewed By: ngraham

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D19932