plugins/slidingpopups: Fix a crash on compositing restart

The sliding popups effect can be unloaded while there's still an active
animation for a closed window.

If that happens, the EffectsHandler::windowDeleted signal may be emitted
when the EffectWindowDeletedRef objects are destroyed. It happens after
the SlidingPopupsEffect destructor.

The sliding popups effect also has an EffectsHandler::windowDeleted
slot.

So, the problem is that m_animationsData gets destroyed first, then
m_animations is destroyed next. When m_animations is destroyed, the
deleted references will be released and EffectsHandler::windowDeleted
will be emitted. SlidingPopupsEffect::slotWindowDeleted will access
m_animationsData whose memory has been just released.

In order to prevent crashing, this change ensures that animations are
canceled while m_animationsData is still valid.