plugins/slidingpopups: Always ref window when sliding it offscreen

Imagine the following case:

- the window is hidden, slideOut() is called but no deleted reference is
  created because the window is not deleted
- the window is closed, slideOut() won't be called because the window is
  hidden and SlidingPopupsEffect::slotWindowClosed() ignores closed windows
  that are already hidden
- the window is deleted in meanwhile
- the sliding popups effects attempt to delete m_animations[w] entry,
  but since "w" is a dangling pointer at this point, visibleRef is going
  to access released memory

To fix that, make slideOut() always ref the window.

(cherry picked from commit 0c03e7cc)