Akonadi::AgentInstanceWidget: don't leak pixmaps

This fixes the rather alarming
  ~QX11PixmapData(): QPixmap objects must be destroyed before the QApplication object, otherwise the native pixmap object will be leaked.
error on KMail shutdown, at least for the non-QWebKit case, though I'm sure there are more where this one came from.

MERGE: 4.5

svn path=/trunk/KDE/kdepimlibs/; revision=1146144