Fade in/out when changing wallpaper

[aiden1999]

Version of Variety you are using
0.8..12-2

Is your feature request related to a problem? Please describe.
I find how the wallpaper suddenly changes to very jarring.

Describe the solution you'd like
Fade in between the two wallpapers that lasts for a second or two - similar to how desktop background images change in Windows when it is set to slideshow.

Additional context
none