Draft: CalendarDecoration::Element: use QIcon property instead of QPixmap
Enables to reuse the built-in pixmap-as-needed-for-size utility of QIcon instead of own code. Also fixes the pixmap/icon support of StoredElement, which missed to reimplement virtual QPixmap newPixmap(const QSize &) and there feed the pixmap it stored.
(Waiting for kdepim-addons!17 (merged) to land, which is done for the old API)