QVariant was using QColor::name() to convert a color to string, which by
default loses alpha value. The patch is fixing the problem by always
including the alpha value in the string when required.
[ChangeLog][Core][Variant] QVariant(QColor)::toString() uses
QColor::HexArgb format when the alpha component is different from 1.
Task-number: QTBUG-37851
Change-Id: I887460c1ea151180ba99d64dd873ba9d6e2268f2
Reviewed-by: Simon Hausmann <simon.hausmann@theqtcompany.com>