Replace manual memory management with unique_ptr and manual
re-implementations of QMarginsF with The Real Thing™.
The only noticeable difference should be that the checks for 0 now use
qFuzzyCompare() (by way of QMarginsF::isNull() and its operator==).
[ChangeLog][QtWidgets][QGraphicsWidget] Added QMarginsF overloads of
setContentsMargins() and setWindowFrameMargins().
Change-Id: I6b3bb87015fa52fdde245b7528cca4b8f9ce41e1
Reviewed-by: Mårten Nordheim <marten.nordheim@qt.io>
Reviewed-by: Richard Moe Gustavsen <richard.gustavsen@qt.io>
ca0c9f82cb