[color-wallpaper] Interlink child and parent form layout

Summary: Should in some odd case (language) the color wallpaper's "Color:" label be longer than "Layout:" or "Wallpaper Type:" we can ensure the alignment between parent and child layout will not be broken.

Test Plan:
This patch ie. alias works thanks to D20313.

Before:

After:

Reviewers: Plasma, ngraham

Reviewed By: ngraham

Subscribers: plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D20965