[Image Wallpaper] Make color picker visible for all positioning mode.

Summary:
If user is using svg wallpaper with opaque area, then this allows users to fill opaque area with color.

BUG: 351012

Reviewers: Plasma, bshah

Reviewed By: Plasma, bshah

Subscribers: bshah, broulik, plasma-devel

Projects: Plasma

Differential Revision: https://phabricator.kde.org/D1008