[Feature Request] Add Wallpaper Fit Options (Centered, Tiled)

[wallh4x]

Hi, it would be great to have additional Wallpaper fit options like Centered and Tiled for images that are smaller than or a different aspect from the base screen resolution. Centered images could also imply a background color (potentially the last color set in the "Color" option for desktop background?)

This is a useful feature I've used in other desktop environments that would add additional fit and finish to COSMIC's customization language.