Add option to render Kurve between wallpaper and desktop icons

[Amaderis]

I would like to request an optional desktop mode where Kurve is rendered between the wallpaper and the desktop icons.

Current behavior:

Wallpaper
==> Desktop icons
==>Kurve visualization

Desired behavior:

Wallpaper
==> Kurve visualization
==> Desktop icons.

Use case:

Kurve can be used as an animated desktop background effect. The visualization would remain visible over the wallpaper, while desktop icons stay above the visualization and remain fully visible and unobstructed.

This is particularly useful when using transparency in Kurve, allowing the visualization to blend naturally with the wallpaper without visually interfering with desktop icons and files.

I understand this may depend on KDE Plasma limitations and the available desktop layer APIs, but it would be a valuable optional mode if technically possible.

[luisbocanegra]

This is in fact possible as shown by my other two projects https://github.com/luisbocanegra/plasma-wallpaper-effects and https://github.com/luisbocanegra/plasma-panel-spacer-extended :)

This is currently being tracked by #30, but now that you created this issue it made me think that it is probably a better idea to keep the "rendering behind the panel/other widgets" and on the desktop as two separate issues so I am renaming the other