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.