Use QColor instead of Qt::GlobalColor in item data

The QVariant constructor that takes a Qt::GlobalColor is removed in Qt 5.
It is actually a kind of special case in Qt 4 anyway. Fix the uses of this
by constructing a QColor and using that instead.
Merge pull request #1288 from simonsonc/qvariant-qcolor

Use QColor instead of Qt::GlobalColor in item data