[effects] Split the Fade effect

Summary:
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
popup windows.

Test Plan:
Have been using the Fading Popups effect in combination with the Scale
effect for a couple of days. Haven't noticed any significant differences between
the new combination (Fading Popups + Scale) and the old combination
(Fade + Scale).

Reviewers: #kwin, #plasma, #vdg, graesslin

Reviewed By: #kwin, #plasma, graesslin

Subscribers: graesslin, abetts, ngraham, kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D16836