[color-wallpaper] Align with the master FormLayout

Summary: This patch aligns the color wallpaper plugin's options with the FormLayout above it by making use of twinFormLayouts.

Test Plan:
D19932 needed before testing.

Before:

After:

Before:

After:

Reviewers: Plasma, ngraham

Reviewed By: ngraham

Subscribers: plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D20048