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.
1 job for !33 with work/kossebau/useiconpropertyondecorationelement in 2 minutes and 24 seconds (queued for 94 minutes)
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