Currently, we have three effects that can be used to animate the
appearing of toplevel windows(fade, glide, scale) and one can enable
all three of them, which seems to be wrong. It doesn't make sense to have
glide and scale effect enabled, for example.
We couldn't put all three effects into an exclusive group before because
the fade effect animates not only toplevel windows but also popups. So,
if all three effects are in an exclusive group and you enable glide effect,
for example, then tooltips and other popups won't be faded in/out.
This patch splits the fade effect into two: the first effect (called Fade)
animates toplevel windows and the other one (called Fading Popups) animates