workspaceAnimation: Handle DESKTOP windows

workspaceAnimation: Handle DESKTOP windows

Windows of type DESKTOP are currently handled like all other sticky
windows, and stick to the monitor while the animation happens in the
background.

This behavior is odd for desktop windows, which are otherwise always
kept underneath regular windows.

Instead, make them part of the background, so they keep the expected
stacking position and animate together with the workspace.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/2865>