color in QML should be constructible from Qt::color enumerations

Currently one can build a color in QML from

• (a QColor in C++)
• a string representing a SVG color name
• a string in hash+hexadecimal (#RRGGBB)
• a construction function like Qt.rgba()

A direct conversion from the Qt::GlobalColor enumeration is missing, although the enumerators themselves are exposed to QML:

Rectangle {
  color: Qt.red // ERROR: no int -> color conversion
}

The error shows that the problem is deeper than a mere "conversion" but has to do with the messed-up state of enumerations in QML (sometimes being integers, sometimes being enumerations).

While at it, it would be nice™ to have the color constants in QColorConstants exposed in QML as named objects, avoiding the risk of typos and the cost of parsing a string.