The glide effect tried to not animate any window the SlidingPopups
effect animates. But it detected that in a very crude way. It announced
the KDE_SLIDE atom and checked for windows having that property set.

This has a few disadvantages:

• KWindowEffects::isEffectAvailable gets confused as an effect announces support for SlidingPopups which doesn't provide SlidingPopups
• The approach can only work for X11 windows
• The approach causes X11 usage in the ctor

With this change the GlideEffect implements a slot for
EffectsHandler::windowDataChanged to detect that the SlidingPopupsEffect
grabbed the window. The X11 atom interaction is removed.