Support Kirigami.twinFormLayouts in individual wallpaper plugins
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
Differential Revision: https://phabricator.kde.org/D19932