splash: Fade out animation
Fading out the splash makes the splash image look more related to the application. On older phoc the animation is just skipped.
We could avoid the animation in case the splash times out.
I'm marking this as draft as phoc!420 (merged) should land first to not use an unmerged protocol.
This has some overlap with https://gitlab.gnome.org/guidog/phosh/-/tree/transparency-css which is nice since with this MR and !1252 (merged) the MR to make the overview transparent becomes a rather small addition.