CalendarDecoration::Element: make property getters const
Second commit adds Q_REQUIRED_RESULT to StoredElement overrides
No changes needed in 3rd-party subclasses, by what I saw.
Kept newPixmap(QStze) non-const, as it needs changes in kdepim-addons/korganizer/plugins/picoftheday and !33 (closed) wants to remove that method anyway.